What you see is gunk coming out of the internal part of the chain. It's a mix of the old lube, dirt trapped inside and the new lube. If you want a perfectly clean chain, soak it in fuel overnight, then rince it with WD-40 and then isopropyl alcohol. It should be 100% clean. After letting it dry, you can go ahead and apply your lube.
My experience with Muc-Off has been great so far. However, I switched from the ''regular'' Dry Lube to the C3 ceramic dry lube this year and noticed that the C3 gets the chain gunkier after each rides than the regular one. I'll switch back once my bottle is empty.