My only regret is that I can't figure out how to get hundreds of them at once without attracting attention and having to explain that I hate the things.
Ah, beat me to it!
Blackbirds are federally protected by the United States under Federal Migratory Bird Treaty Act. They may only be killed when they are found committing or about to commit depredations upon ornamental or shade trees, agricultural crops, livestock, or wildlife, or when concentrated in such numbers and manner as to constitute a health hazard or other nuisance, as stated in federal laws regarding migratory birds (50 CFR 21). Additional blackbird killing restrictions will vary per state.
http://www.crittercontrol.com/facts/animals/blackbirds.html
Go to your local machine shop. Get iron filings. Mix with starling/grackle bait. Place near the infestation.
Don't expect instant results. The local population will drop, they dead birds won't all be in one place, nor will they drop at onetime. Toxicology tests, should anyone be so motivated, will be negative...