I don't know if this was intentional or not, or if it's meant to be smelted in a different type of furnace that I don't know about, but netherrack can't be smelted in blast furnaces. You can still smelt it in regular furnaces, but it would be more fitting if you could also smelt it in blast furnaces. If this was intentional or not, thanks for reading this.
Blast furnace is only for smelting ores and excess iron and gold tools/weapons/armor.