Do you just mean that you read a 21-step wedge that was made with an existing profile or just making a single channel print of a 21-step target (black only) and measuring that.
It is great that the STS matte black is giving such good dmax. It is much better than what I am measuring with both Cone and MIS matte black, which top out at 1.65-1.7 with most papers. What will be interesting is seeing the rate it reaches the dmax before leveling off or reversing density.
After looking into it a little, it seems that the matte black referenced is the same as (or interchangeable with) what is used in the epson K3 inkset? I went to STS website, and all that is available there are prefilled carts. Investigating further makes it nearly impossible to understand where all these off brand inks are coming from.
The blotchiness I've noticed with the current MIS inks is that it tends to clear up as it dries—especially around the 50% mark with a full 6-dilution profile. The exception is for the single black channel around 0-10% density, but I would think this is probably from dithering more than anything else.