The block size is almost certainly 512 bytes.
Which is what I said - the block siize is set by the HW.
But ... the issue is trying to get the TK-50 to stream. Hence the traditional unix: dd ibs=64K obs=XXX | tar xvfp - trick.
This will tell the driver to read upto 128 blocks in one DMA and then pump the bits into tar a 'XXX block' at a time (which is is usually 20b for tar/cpio/dump et al and Ultrix obey's).