The video below is a clip of blocking the concave curve of a fender. Notice the main area of the fender, the top section, is already blocked. the curve needs to be blocked into the area, rather than starting with the curve, and then trying to make the main area match the curve.
Choosing the correct block is very important. The block being used is flexible, but just enough that it conforms to the curve without digging in. Notice, with True Blox, the handles are able to be used to pull the block ends up, creating the perfect arch.