Railcraft's Coke can be used in non-Railcraft Blast Furnaces, but Railcraft's Block of Fuel Coke cannot. As I understand the problem, its related to how Railcraft's oredict entry for the block (oredict: blockCoke) differs from the usual (oredict: blockFuelCoke)

I assume adding the oredict entry blockFuelCoke for it may solve the issue.