Sure, but there's no reason to assume he can't use it. This would be like assuming that the Issho can't use Ryuo or Dragon can't use hardening because he hasn't shown it. If Whitebeard (whose strength derives mostly from his Devil Fruit power) was capable of that level of flow control, Mihawk probably was as well.
Roger could have had superior COO to Mihawk and probably had the ultimate form of COC.
- Current Garp is old and retired (hence significantly weaker).
- Even after the Rocks fight, Garp needed to train to confront Chinjao.
What
@trav said is that it's speculation that Mihawk turned Yoru black, not that it's speculation that COA is what blackens swords. We already know that's how black blades are made from Mihawk.