It's absolutely not too late to put up nest boxes! Bluebirds and other cavity nesting birds typically have more than one brood per season and can switch to a new site for their second or third brood. Or if their first nesting is unsuccessful, perhaps due to predators, the birds may look for a better nest box.
You can put houses up year round, actually. Some birds will use nest boxes as roosting sites in the winter. So you may have missed the first batch, but are just in time for the second or third!