Initial thoughts after playing it a little bit at bedroom volumes:
The controls seem to interact with each other a lot. Whereas boosting or cutting treble or bass on my Darkglass Vintage Deluxe only really effects that frequency, those controls on the Mr. Black seem to change the character of the whole sound. i.e. boosting treble seems to thin out the low end a bit, and cutting treble seems to fatten the low end a bit. I've also noticed that yes, with everything flat and the gain anywhere above the lowest 10%, the low end is noticeably thinner. Boosting the bass control definitely brings it back and can boost a little past "flat". However the knob doesn't seem to be linear, in that it doesn't have much effect until the last 10% of the turn, and then it's very powerful.
I don't have experience with a lot of dirt or preamp pedals, only the sansamp, vt bass, tonehammer, B3K and Vintage deluxe. This to me sounds more like the VT bass/tonehammer overdrive in that it is more like an overdriven amp. The notes below low E seems to lose some clarity when the sound is really broken up (above 9 oclock on drive). We'll see how it sounds at full volume though, it might sound good. The Darkglass offerings do a really good job of keeping that clarity at all ranges and at all points on the drive knob so they are definitely different there. I do like the sounds that it makes though, so I'd like to make it work with my Drop C band.
Something I wasn't expecting, was that this pedal does a pretty nice job of smoothing and fattening my PB5. In passive mode, those pickups are very treble and high mid present. I'm always turning the tone knob down a bunch and trying to EQ to the sound I want fingerstyle. With the gain on the Mr. Black down almost to the bottom, the bass boosted to 3:30 and the treble cut to 10:00, it has a nice fat sound that still has a tiny bit of treble but is more bass and low mid forward without being muffled. I A/Bed it with the tone wide open and the pedal on, and then the pedal off but the tone cut to about 2/10 (where I usually leave it), and it sounded muffled in comparison with the pedal off. It also still adds that hard to describe 3 dimensional quality that you get with faint overdrive, but I honestly couldn't hear the tone breaking up. Pretty cool.
So first impression summary is that I wouldn't use it as a "preamp with the bonus of having OD", but as an OD that can give you some very nice and unique sounds with tweaking.