Yes, the crud is normal. Next time before you start scraping, let the end of the barrel soak overnight in something like automatic transmission fluid or Kano Kroil, and follow-up with engine degreaser and a brush afterwards.
If you want to remove an older flash hider, first determine whether it screwed on with a standard left-hand off thread, or a post ban model which may be pinned, soldered, or press fit in place. (If this is the case, I'd let an armorer remove it.)
If it's threaded on you're going to need a strong vice, vice blocks, and a 3/4" armorer's wrench. (Brownell's or Bushmaster has these items.) There should be some sort of compression (crush) washer behind the flash hider; and, you'll need to install another one on the new installation. These things are usually torqued back on with no more than 20# force.
Since you're going to all this trouble, be sure to get a flash hider that doesn't have an open port on the bottom - That's a real nuisance when you're firing prone.