to me. Take it for what it's worth (very little). It's not rocket science.
That said, Davis was lined up against Blackmon several times during 7 on 7's and Davis had no trouble staying with Blackmon whatsoever.
Blackmon reports a 10.5 second 100 meter but I cant find any evidence that he ran that sort of time in any organized competition (admittedly, my search for his 100 meter times has not been exhaustive or comprehensive). As near as I can tell, he's a 10.7 type guy (yes, that's very fast...especially by BYU standards). Conversely, we know that Davis has run the 100 meter in the mid 10.6's in organized competition.
Several coaches and players have said that Davis is the fastest guy on the team. This leads me to believe that Davis is, in fact, the fastest guy on the team. No slight to Blackmon. He's very fast, too. I suspect the two are only separated by a tenth of a second (or less) in the 100 meters.