Welcome. You might just get lucky on the headspace with a current Blackhorn 209 compatible primer. I would not use the Federal Muzzleloader Primers for Blackhorn 209. They may work in warm moderate temperatures, but will come back to bite you in colder temperatures during seasons. The Win W209, Fed 209A, and CCI 209M (Magnum) primers would all be good choices.
BEFORE all that, you'll need a Blackhorn 209 compatible breech plug, as the OEM was designed for pellets and easier to ignite loose powders, and is not reliable with Blackhorn 209. There are a several options available now for Blackhorn 209.
CVA BLACKHORN Breech Plug.
https://cva.bpishopping.com/shop/blackhorn-qrbp-accura-optima-wolf.html
Better price, and a sponsor here. Muzzle-Loaders.
https://www.muzzle-loaders.com/accessories/cva-qrbp-blackhorn-209-breech-plug.html
I believe I'm really going to like this site. Sure do appreciate all the info Busta from you and the other gentlemen. I' hope to help others also shorten their learning curve. I bought the BH breech plug from muzzle-loaders same time as other items. I do recommend them for customer service and help. Never tried Blackhorn before (price). I only used percussion before. Thanks again.
Western Powders Blackhorn 209 Breech Plug.
https://www.westernpowders.com/index.php?route=product/product&product_id=147
Lehigh Defense / SSK Vent Liner Breech Plug. Comes with one vent liner.
https://sskfirearms.lehighdefense.com/all-products?product_id=308&sort=p.model&order=DESC
Hankins Direct Ignition LRMP Breech Plug. Rear Sealing.
https://hanksprecisiongunparts.com/p/cva-replacement-plug-for-the-accura-optima-wolf
Arrowhead Sporting Goods Gen 2 LRMP. Rear Sealing.
https://www.arrowheadrifles.com/product-category/breech-plugs/
Several choices for every budget, and beyond.
If you decide to go with the o-ring, give me a holler. I came up with the idea back in the summer of 2008, kept it to myself for several months and through the hunting seasons, then shared it with everyone in January 2009. Was thinking about looking into a patent, but never really researched it. It's all PRIOR ART now, and has been for 11 years. It is very effective, sealing out the elements, and eliminating blowback. Just need to adjust your headspace with your firing pin bushing, just like you will probably have to for a bare primer anyways, unless you like leaky primer pockets.
Go here and read everything. click on all the blue tabs, especially everything under TECHNICAL, LOAD DATA, and FAQ's. It will shorten your learning curve immensely. There are NO shortcuts when shooting Blackhorn 209, so don't try to cut corners, or it will come back to bite you.
https://www.blackhorn209.com/
And don't be afraid to ask questions here. Several of us have been shooting Blackhorn 209 since it was first commercially available in early April, 2008, 11.5 years ago. Where does time go?