Daniel, TT, and JJ run around 4.5. Davis runs about the same as Blackmon.
Most years, your inference based on BYU DBs might be right. This year, you are very wrong.
Blackmon won't see many DBs who run sub-4.5. Texas for sure, maybe Central Florida, Virginia, and one or two others. He'll see a lot of guys who flat out won't be able to keep up with him, though.