A "blitz", by definition, is rushing more defenders than the offense has back in protection. So when there's an empty backfield, the offense has the minimum of 5 players in pass protection (the OL), which means the defense has to rush a minimum of 6 players to create a blitz.
For obvious reasons, it's very risky, and it's not something Diaco's system is designed to do. One of the strengths of the 3-4 is that it allows the defense to effectively pressure the QB without resorting to extremes like dropping DL into coverage (zone blitz) or bringing the house.
Rushing 5 and dropping 6 is a very common practice for 3-4 defenses on passing downs. As was mentioned above, I expect to see some combination of Ishaq, Tuitt, KLM, Shembo and Day as our 4 DL, with Diaco bringing pressure from one unexpected source-- most often a LB, but occasionally a DB.
This isn't a true blitz. Against most offenses, our rushers will probably be outnumbered by a RB who stays in to protect along with the OL. But if our 5th rusher doesn't give away his intentions pre-snap, the randomness involved gives us a good shot at sack without risking giving up a big play, because we'll still have 6 defenders in coverage.