Unfortunately black knot is very common disease, and easily vectored in by insects. You can protect the tree against insects by giving it a ground drench of Merit. This may help, and also make sure you don't do any pruning until the late fall, after the insects have died off.
However, if your tree does develop the disease, prune off any infected branches immediately! The sooner the better. If you can catch it while the galls are still green, you stand a better chance of the diesase not spreading as rapidly.

Do not dispose of infected materials (including leaves) in a mulch pile) It is best to burn it or if not possible at least dispose of it in the garbage.

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Mike Maier
aka
The Plant Doc

Lilac is the 1st that comes to mind, Magnolia would be another. Crab and other ornamental apples make wonderful displays, although they have been getting hit pretty hard recently in this area by anthracnose (a fungal disease, but not as much of a death sentence as black knot.
There is the Bradford pear, and also the honey locust. All which have wonderful floral displays and would be hardy for your area with the possible exception of the magnolia, I am not sure what zone Page is.

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Mike Maier
aka
The Plant Doc