I remember a guy shaking all the berries from his many dogwood trees onto sheets then putting them in plastic milk jugs. Can't remember if he froze them or just stored them in an extra refrigerator but through the winter he would put these out mixed with a few raisins and he had scads of bluebirds.

Here is a link that might be useful: Bittersweet Gardens

Do a google on bluebird feeding and you will come up with several recipes for winter feeding.....many include peanutbutter and suet, etc.

Here is a link that might be useful: information and recipe

I've had good success with something much more pedestrian- hulled sunflower in a basic A9 feeder. I've been using hulled sunflower for years to avoid the mess of regular (or black oil) seeds. The blue birds discovered it and are right there with all the other birds, especially now in the winter, to clean my two feeders out in a matter of days.
Bill