Wow, I guess you bought these awhile ago, since there hasn't been a Grade B since 1975. Okay, here is the skinny on your blocks...
Grade B went away in 1975 when GGG-G-15B was superceded. It was replaced by GGG-G-15C, which stated that Grade 3 is the lowest grade used in the U.S. (for gov't service, actually, which is why these fed specs are written). Grade 3 is what they called 'a compromise between Grade A and B. Off the top of my head, I can't remember the tolerance for Grade B (it was somewhere in the 12-16 µin range as I remember), but what your vendor is correct, Grade 3 is (soon to be was) the replacement for Grade B, and the tolerance up to an inch is +8/-4 µin. There is no American specification for ±50 µin, although it is a common "Economy Import" set tolerance, common enough that we used to cal to it and called it a "Workshop" Grade.
Here is the where the REAL fun begins. GGG-G-15C was cancelled a few years back, and was replaced by ANSI B89.1.9, which thankfully kept the same tolerances. But, the final touches on the NEW B89.1.9 are almost done, and the NEW version uses the tolerances used in Europe (to facilitate global trade, yada yada yada). The NEW tolerances look NOTHING like the old tolerances.
So what should you do? Is ±50 µin good enough for your process? If it is, then specify on your P.O. that ±50 µin is the tolerance you want used. Every quality system allows for you to set tolerances on gauges to meet your needs, and in this case, you probably should do just that. Your vendor should have no problem with a custom tolerance, and if they do, then let them know that there are a lot of very good quality calibration vendors that do gage blocks.
Good luck,
Ryan