No reason to apologize, I recognize you have no experience with the actual gun and were speculating. I just thought I'd throw out my two cents based on my personal experience with the particular gun being discussed.
The manual makes it clear that powders proofed as smokeless are a no-no. Black Powder (the real deal), Replicate Subs of Black Powder (Pyrodex or 777), or Non-Replicate Subs (such as Blackhorn 209), would be mandatory. Blackhorn 209 is not a black powder replicate substitute, such as Pyrodex or 777. The replicas are designed to function just like black powder, i.e., load, smoke, water clean-up, etc. Blackhorn 209 is a non-replicate powder, used with nitro solvent and oil, that is proofed and classified as a black powder substitute. Western Powders has proofed this non-replicate blackpowder substitute for a maximum load of 120 grains by volume. They immediately tested the new Remington with this maximum proofed loading, and it was clearly evident that the gun is far more capable to be pushed further, hence, their announced project to work with Remington to develop a new load manual. As long as you stay within their maximum recommended guidelines, you would be safe.
As for replicate black powder subs, the only current recognized duplicates of the real deal are Pyrodex and 777. As for 200 grain volume loads, the manual is specific to a particular granulation or pellet loading by volume of 777. Blackhorn 209 is more energetic, so loading 200 grains by volume of Blackhorn would be higher than published by Western Powders and would be outside a proofed and recommended maximum loading. To the best of my knowledge their last bit of information put out on this subject was shooting 150 grain volume loads, which is thirty grains higher than their initial max load testing. Pretty certain that "Remington Ultimate Only" data should be published before too long.
My particular rifle did have a few bugs needing to be worked out, but it is running exceedingly well shooting nothing but Blackhorn. I don't get caught up in the hype over firing 200 grains in pellets, 209 vs LRM primers, etc., but I do understand the industry debate over successfully burning all four pellets. To my knowledge, no person has designed a 209 breech plug that was capable of using the superior fire power of the 209 primer to ignite all four pellets, because all 209 primer modules have an inherent spacial void between the primer and the fire channel of the breech plug which kills efficiency. The Johnston breech plug re-designed by Remington is able to maximize the fire from the LRP directly into the fire channel, having an end result of a stronger ignition firing into the load. It is clear that the new Remington does ignite all four pellets, which was not done before with the 209. So, that is the hype Remington is marketing via the 200 grain 777 pellet loads. But in reality, the best performance you'll find out of this gun will not be from those 777 loads, but will be found shooting Blackhorn. This has been repeatedly proven countless times since the gun hit the shelves.
Best
