I'm not so sure about this being a Blackpoll.
Just curious, what is gained by actually assigning names to these things?
These disheveled mounts are essentially useless as an educational or research tool, and even more useless with guessed/consensus species names assigned to them. Without data, they're not specimens. They're just dead birds.
Hummmm, I'd be curious as to why you do not feel this is a Blackpoll? Listing the reason why would be the only way to properly understand what you post.
As to the value of these old mounts, there are many uses and most importantly, we do not know what value they could play in the future. As long as we have the room to store even these, as you called them "disheveled" mounts, we will preserve them for future use if needed. As to their proper ID, if there is a doubt, they are not labled, but notes remain with the specimen. Never just "dead birds".