Block both flat areas first, then sand the radius. As a rule you block the larger surface areas first then blend in the details. A soft or hard block can be used depending on if there is any irregularities that need to be removed. Hope this helps.
This is the reasoning: if you were to do the radius first then the flat areas a line would show at the edge of the flat areas where it turns at the radius. Guidecoat can prove this theory. Bob