Let me expand Mr. Sample's post.
When System/360 DASD was originally defined device capacities were rather limited. The IBM 2314 disk device, for example had a track capacity of 7294 bytes, if I remember correctly. The most effective use of these devices could be achieved when the BLKSIZE was close to 7294, so 7280 (INT(7294/80)*80) would be "optimal" for the 2314.
By the 1980s devices could store more than 32768 bytes on a track. Since data management has a restriction of 32760 bytes in a record, full track records were no longer possible; so the decision was made that an "optimal" record size would be defined as whatever value <= 32760 that permited 2 records on a track. As Mr. Sample says, this works out to 27,998 bytes per track for 3390 type devices, even the simulated 3390s now commonly used, and data sets containing 80 byte card images should use BLKSIZE=27920.
No reasonable person should have to memorize this garbage, much less do the arithmetic, so the system will automatically figure this out for you when you do not specify a BLKSIZE.