I've never found any actual evidence of that. Granted, I haven't spent hours looking for official times, but, I couldn't see anything but his own claims to fast times in my cursory google searches.

Sub 4.4 40 yard/10.5 sec. 100 meter guys don't get run down by safeties in the open field when said track star has a 10 yard head start.

I'm telling you, if Blackmon was ever as fast as he claims, he isn't now. Not on a football field with pads on, anyway.