I would think programming something from scratch with C# and Managed DirectX is more credible than using Blitzmax.
Credible to who?
A person who thinks C and their hybrids are the only credible languages in the whole wide shiny world :)
If I were to make some standalone tools for a 2D game being made on another engine (like Torque Game Builder), which would you recommend I use more? Blitzmax or C#?
I'd use the language you're the most comfortable in. But if you were planning on releasing the source code to the "Torque" community, then I'd research which one would be more appreciated.