Yep, worked great!!!! Grey stock now totally black, just about perfect match to VLTOR/Magpul black, almost kinda a satin-y sheen, three rit dye packs in simmering water + white vinegar for about 45 mins. Grey rubber buttpad, no effect whatsoever, ha. I had been trying to trade my stock on the EE with no bites, didn't want to buy a new one. $4.98 Wok at Wal-mart + $2.68 per pack of rit dye + free white vinegar from my pantry + very confused looks from my kids + very questionable look from my wife when i took over her stove = black B5 stock.
I'll post before and after pics if anyone cares, if it would help anyone else out. I'm going to do an FDE MOE grip next, had I remembered at the time I would have done it alongside the B5.
I was reading about some yo-yo (yes, yo-yo) collector dying various plastics, and he uses acetone along with rit dye on certain types of polymers. Had never heard of that before but I guess on certain types of polymers the acetone is what opens up the pores.