If I include an asset from a Mod inside one of my Blueprints and then post it, will it be usable for others who download the Blueprint?

I assume that I will have to put in a disclaimer that the Blueprint has a dependency to Mod XXX and that you must have the Mod installed inside your game.

But since Blueprints are just a scramble of Asci characters that you copy and paste, how is the foreign Mod Asset coded inside the Blueprint?

only the modded machine/equipment won't be usable, but the rest of the blueprint will be usable, it will show you warning there in the blueprint though.

easiest way to check it is by make your blueprint with modded machine, save it. exit game, open game, start sandbox with no mods, then check that blueprint, you will get a warning that machine isn't available.