Plenty fast for safety. Lets keep in mind Polamalu runs sub 4.4. He's a complete freak.
Last years NFL combine results for Strong Safety:
40-Yard Dash
1. Earl Thomas —Texas (4.41)
2. Eric Berry—Tennessee (4.47)
3. Reshad Jones—Georgia (4.54)
4. Justin Woodall—Alabama (4.59)
5. Larry Asante—Nebraska (4.63)
http://bleacherreport.com/articles/359746-2010-nfl-scouting-combine-strong-safety-results
Here's the complete list:
http://pickingpros.com/blog/nfl/nfl-combine-2010-40-times-verticals-etc-–-full-list/
If Blacken can shave a few tenths of a second off of his unofficial time then we are looking at elite D1 speed.