Half BAB because we took the Incarnate as a basis so we wanted some abilities that gave him consistent bonuses if you tried hard. The class is intended to be a good melee only with serious effort and hey, there's a +1 bonus that gives you Medium BAB.
The spices you get are listed in the main class table, along with the BAB, saves, and the maximum blend level available.
Blend enrichments: we wanted a bonus that was uncommon but not too uncommon. We thought about alchemical but, there's basically nothing that gives an alchemical bonus in 3.5 to that would have been the same as an untyped bonus and we thought that is was a bit too strong. About pronouns, I'll read through the text and try to fix it.
Mistwalker: we wanted to prevent the user from flying 24/7 as long as he could spend an action in 5 rounds. This way he still has to land. Suggestions?
Shadowsmoke: Greater Mirror Image is, IMO, stronger! It doesn't even need an action to replenish the illusions.
Umbral weapon and siege weapons are an obligatory reference to The Gamers. Feel free to disregard.
Diamond Filter is the material component for Revivify, which is the blend's "Unleash the smoke" power. For fluff reasons, you incorporate it into the blend and "burn" it if you use the UP.
Deep Voyage Dust: no, but there is the usual range (60 feet).
Gear change: since this is a blend first gained at level 16, I don't think any magic item is going to be really really OP in a game where T2s are allowed. With this premise, I'm quite sure that any level 16 character will have at least a 36k item, AKA a +6 bonus to one stat. 36k seems high enough for the power to not be UP, in my opinion. About the McGuffin, yeah. As a DM I'd price a McGuffin at 0 gp to make sure that the party doesn't sell it though
Thanks for the appreciation! And thanks for the formatting!