When you use a brush on a Fossil Block, you would get the brushing animation, you would see the covered fossil sticking out of the block, and eventually the Fossil Block would convert to stone and the fossil would be dropped. Just like how it works with suspicious sand and gravel.
I think this would be good for immersion with Minecraft, as Fossil Blocks look as "suspicious" as the suspicious sand and gravel. Plus, brushes are one of the tools used in real life to uncover fossils, so it makes sense for them to be involved in obtaining Pixelmon fossils.