Among all MLB starters with 70 or more innings, five have a K/9 rate less than 4.5. The ERAs for that group (including Blackburn) are 4.49, 5.18, 7.33, 4.58, and 5.52. All of them are groundball pitchers to one degree or another, whereas Hermsen currently profiles as a moderate flyball pitcher (batters hit the ball in the air more often at higher levels of play). It is literally impossible to succeed with that combination.
But the even bigger problem is that 4.5 K/9 in the Majors is a lot different than the same rate in AA. In other words, Hermsen would actually have to improve a lot just to have a small chance of being a short-term 5th starter, which is basically a worthless commodity anyway. This is the same issue as with Herrmann, who is not really a prospect because back-up catchers are easy to acquire.
There are finesse pitchers that improve on their ability to keep hitters off-balance. Doug Fister is a current example. It is pretty rare, though. There are dozens of pitchers in the system who could make unusual strides- and most likely one or two of them will. My point is just that nothing really separates Hermsen from a bunch of other 'C' prospects that get little attention.