There is football speed and then there is track speed. Blackmon runs very tenetively, which probably negates his "athleticism"- for lack of a better word. Even though I've never seen him at top end, I'll take him and the various recruiting services at their word that he IS fast. If nothing else, his scholarship to Oregon is proof enough that he is fast.
That being said, he did get walked down pretty badly. For Juergens appearant lack of athleticism, I don't think he would have gotten caught from behind under the same circumstances. Lots of CBers were clamoring for more Blackmon PT, but I can see why the coaches lean toward Juergens.
It's the same conundrum between Ross Apo and Cody Hoffman. Ross is the one built with the NFL speed and athleticism, but for a number of different reason, Hoffman was the one making the plays in the games. You look at Apo and wonder how he isn't more of a reliable threat, but it comes down to the reality at some guys are gamers and some aren't.