Can a blank revolver be converted to a firearm by drilling the barrel and replacing the cylinder?

First off, it's highly illegal. Second off, it's highly dangerous. Blank barrels are a much lower grade of metal than the barrels used in projectile emitting firearms, because they're not designed to have projectiles going through them. Blank revolver cylinders are typically proprietary, and do not have the same dimensions as other revolver cylinders, so you'd have a hard time finding one which aligned properly, as well.