Infestations by "big, black ants" often are a sign of a tree that has already been weakened by other damage.
Have you ever been out digging in your garden and suddenly been surrounded by an odor of lemons? If so, you have probably disturbed a colony of larger yellow ants. When disturbed, ants commonly release a defensive compound that is irritating to predators and warns other members of the colony that there is a problem. In the case of larger yellow ants this compound smells to humans very much like citronella or lemon.
The description of “"big, black ant" nearly always refers to carpenter ants, even though other species of ants are nearly as large and black, also. Carpenter ants are much feared, usually more so than is warranted. On occasion, carpenter ants do ruin window frames, door sills, deck boards and other things made of lumber. However, in almost all cases of severe damage there is also a persistent and long-standing moisture problem. Too much moisture in lumber leads to both wood decay and carpenter ants. It's hard to say which lets you down faster.