From what i have read these allow you to use large rifle primers it just an insert that fits into your normal 209 primer breech plug and you insert a large rifle primer into this insert
buckbob is correct in that the Variflame allows you to use large rifle primers. The mention of Blackhorn209 in what you've read is simply stating that the Variflame ignition can be used when shooting different powders including BH209.
But the 'adapters' that the large rifle primers are pressed into will not fit into a stock breech plug. The stock breech plug must be drilled out for the LRP adapter to fit into or you will have to buy the breech plug that comes with the Variflame system that is already drilled out.
In picture note the larger hole in breech plug on left for primer adapter to fit.
The primed LRP 'adapter' itself is also shown on the left as well right below that breech plug.
For comparison, the breech plug on the right is the stock breech plug and regular 209 primer.
Also note that the Variflame 'adapter' shown on the lower left of the pic already has a LRP pressed into it and ready to be used. Once primer is used, the primer itself has to be knocked out of the adapter and a new primer pressed in requiring additional,special priming tools to be purchased for seating primers into adapters. Again, the primers press into adapters tightly and cannot be put in with fingers.
IMO, this is just too much hassle and if I were considering changing up ignition systems, before I went with the Variflame route, I would go with the system that utilizes primed pistol brass as a primer.
Also, I believe the Variflame system is recommended for shooting loose powder...not pellets. And it is also recommended that if using the stock breech plug, when drilling out the primer pocket to accept the adapter, that the flash hole be drilled out as well to .034.
As far as your breech plug getting dirty, I know you are using Pyrodex pellets, which Pyro is very dirty, but what 209 primers are you using? Some are dirtier than others. I've found that the 209 Remington Premier STS primers to be a bit cleaner than the Federals,CCI,Fiochi or other primers I've used.
But they all seem to require using a 1/16" drill bit of the flash channel and torch tip cleaner in the flash hole for best accuracy after 6-10 shots.