You can get hole reducers, which make the nest hole too small for a blackbird to enter.

Also, if you have feeders up, take them down so as not to attract blackbirds.

Hole reducers work well. If you have a bird supply store near you (Wild Bird Crossing or other type), they sell metal hole reducers in varying sizes that you screw onto the holes of your birdhouses to keep the larger birds out.

I just happened to find a good link online, though maybe you've seen it (below).

I know how heart-wrenching that is to see a bird carry-off a smaller bird or other critter, especially one you've been feeding!!

We sometimes have a redtailed hawk around that goes for the squirrels and chipmunks. Fortunately, we also have crows, though, and they sometimes chase away the hawk.

The hawk is beautiful, but, I wish he'd dine somewhere else!

Here is a link that might be useful: http://www.wildbirds.com/feeding_blackbirds.htm