As Paul W pointed out correctly, the TK50 and its children in the DLT* family all used a fixed format 512 byte blocks on the tape.    This cannot be changed.   The tape format is handled by the tape controller microcode and all of the blocks on all the streamers (DLT, 1/4, 1/2", 4mm and 8mm) that I know about were fixed at 512 byte, although the OS could write multiple (N) blocks at a time to the tape controller which will will write as N blocks on the tape.  

This was different from 1/2" parallel 7 and 9-track tape which was actually had variable size 'blocks' in the writes and the tape format.  Since the terminalogy was defined (by IBM) for 5/7/9 track drives, we still use that terminology.

The trick is that streamer** format write* (which is bit seral):

                 <HW HDR BLK1 bit serial><512 bytes of serial data [blk1]><HW TLR BLK1 bit serial><HW HDR BLK2 bit serial><512 bytes of serial data[blk2]><HW TLR BLK2 bit serial>.....<HW HDR BLKn bit serial><512 bytes of serial data>[blkn]<HW TLR BLKn bit serial>

The key is that there are no inter-record gaps (IRG) between the <HW TKR BLK x bit serial> and <<HW HDR BLK x+1 bit serial> frames when recording on a streamer.   BTW: they usually use a serpitine scheme - starting with the center of the tape and moving outwards in a circular pattern - IIRC down on tape end and up on tape start -- but that's fuzzy in my memory and I'm at work so I can not look in any of the controller books  have at home.  If you lose the bit stream on input (data under run), the tape controller backs up the tape and when it starts to write again, it goes over the last block trailer and start its new write at the end of it.   For instance the original 1/4" QIC format wrote 4 passes, then later when the recording head got better, it wrote 9 passes and then even more, but in the newer formats (and withe better media) the head was smaller.

Also remember that EOT is handled different in the streamer formats from 1/2" 7/9 track and IIRC EOT can even differ between the different streamer formats.

If you look at 1/2" parallel 7/9-track (which is where the terms and basic concepts originate) 9-track has a 'inter-record gap' between the last block's trailer and the next block's header.  When IBM originally defined that 7 and 9 track formats (whch ANSI later codified), these gaps are defined so that the there is time to start and stop the motors (somewhere, I have a very old IBM document from the late 60s that describes this very well using IBM terms like LRECL and DASD - direct access storage device ;-)

The key difference from a streamer tape is that the IBM LRECL or logicial record size, could (and did) vary ***.  But to try to keep the amount of wasted space (i.e. least amount of inter-record gaps), different programs use different 'block size' and some formats (like ANSI labeled tapes) the block size (LRECL) can vary within the tape itself.

Also, I don't think I ever knew why, but for some reason IBM's tape utilites tended to like LRECL 10240 and 20480.   Since many of us UNIX folks came from IBM and Multics, we also used the same sizes (i.e. 20b or 10240 8 bit bytes) - it was reasonably efficent (we got 150M per traditional 2400" 1/2 tape at 6250 BPI - you could get 1/3 more space when 3M created a 3600" that fit in the original 1/2" reel) . 

   

Thus the on-tape format of 1/2" (which is parallel encoding and one pass over the tape):

               <HW HDR1><LRECL BYTES of BLK1><HW TRL2><IRG><HW HDR2><LRECL BYTES of BLK2><HW TRL2><IRG>......<HW HDRn><LRECL BYTES of BLKn><HW TRn><IRG><HW EOT HDR>[if the last last 'file' on the tape a second<IRG><HW EOT HDR>]

Note:   LRECL BYTES of BLK1 did not have to be the same as <LRECL BYTES of BLK2> much less <LRECL BYTES of BLKn>

Thus concept (and term) of 'tape blocks' was born.  Also note be careful the term 'file' has specific meaning to a tape.  DEC started to use the term 'save set' to disamiguated it BTW.   A tape 'file' N tape blocks, followed by an a EOT mark.    Thus, two adjacent tape marks actually delinated end of recorded data in the tape. Thus in 7/9 track formats when a new file is written the last <IRG><HW EOT HDR> is backed up over and data frame writing starts over writing the second <HW EOT HDR> after the last <IRG>

So ...   what this all means is that from the OS side, you start a DMA on X blocks and then let the tape controller read or write it.  No matter the number of blocks you write on a streamer, it will always write it as 512 byte blocks (similar to how a disk works when set up in 'fixed' formatting).

One more thing to be careful about...  people also talk about 'ANSI tape' format.   This usually refered to the SW format of the data blocks on the tape.   UNIX's native tape formats were tp/stp, tar, cpio and dump. VMS uses the ANSI tape format as its native format under the covers (and if IIRC, so does RT11) for how to write and exchange data - which BTW, originally using those variable LRECL blocks on the tape.

So the undustry first had a define a set if physical encoding for the tapes and these are also ANSI specs.   But you need need to define how the data itself is written (which byte encoding ASCII vs EBCIDIC) and how to understand the 'files' on the tape itself (this is usually what is being tape about when people talk about 'ANSI tapes.' My old housemate at UCB (Tom Quarles, also known as the author of SPICE3) wrote the UNIX Ansitape program that went out with BSD (he wrote so we could exchnage tapes with the DEC CAD team which used VMS).

Clem

* Just to confuse you more, TK50 and the DLT family actually use a 1/2" media in the closed tape cartridge.  But when DEC developed it (with 3M), there were also 3rd party 1/2" tape controllers that wrote bit serial (streamer) format on the traditonal 1/2" (9-track parallel) media.   For instance the USAF/AWACS planes used to use a traditonal 3M 1/2" tape >>media<<, but those tapes can only be read on a special streamer drive [long story - I can make a couple of HW & SW guys shutter when I just say the word 'Grumman' ****].

**  One other thing to confuse the world is that 'streaming' was a trick performance trick that originated with 1/2" tape.   You will see many 1/2" drives from the period such as ones from Cipher and Kenndy that took a parallel byte stream and wrote/read them - although they obey the 1/2" format rules on the tape itself.

***  Another thing that was undefined in the ANSI tape specs and you can sometimes see, but certain HW will toss cookies and not read if you try it, it mix encoding within a tape (i.e. write 800 BPI, 1600 BPI or even 6250 BPI on the same tape).   This was sometimes done on things like boot tapes because the pre-boot system might only know about 800 BPI tapes and it simplified the boot process particularly in the days when you had to toggle in the boot (or in IBM terms -- IPL -- initial program load -- code).

**** An airman on the AWAC used to spent his entire time on the flight keeping the 3 drives loaded - it the time it took to write one tape, another is being rewound and the airman put a new tape on the 3rd - making that all work at full speed with no data loss was 'interesting'