If it's very, very cold, the combs could be getting spots of frost bite, but if they look black, it's most probably dried blood from them pecking each other. That's a very common spot for them to peck each other. Mine have spots all the time from this.
But if it's below freezing, vaseline may help. But be warned, it will then make their heads greasy and it'll make dirt stick. I vaselined all mine up real good during a sub-zero spell, then had to go back later when it warmed up and shampoo all their heads.