The silver maples in our front yard are developing black spots on the
leaves. Not knowing any better, I'd guess this is some kind of
fungus.

Black leafspots on maples that are due to fungal infections (anthracnose, tar
spot, Phyllosticta leaf spot) are usually only seen in the spring following a
period of wet weather. Fungal problems can occur at other times if there has
been a period of unusually rainy weather. They are not a great cause for
concern as they cause minimal damage to the tree.
sed5555