I use Bragdons Weathering Powders, which are excellent I might add and stick like poo to a blanket, but tried using regular chalk rubbed against sand paper prior to the Bragdons. What I discovered was it didn't stick to anything terribly well and when a clear coat was applied, that tended to "blow the chalk" around. Because it didn't stick, it didn't matter what color I used, the color didn't stay/show up.
The other (almost opposite) issue I discovered was when using chalk to replicate mortar lines between the bricks. One of two things would occur when giving it a clear coat - the clear coat would blow the chalk around and cause it to clag up heavily in spots across the wall.
Now, using chalk powder with baby powder might be better, I really can't say but can;'t imagine it improving the adhesive qualities of the chalk either.