So I have hit level 55 and want to prestige but I don't know if I lose my black market tier weapons that I have unlocked.
You will not lose your Black Market Weapons they remain unlocked. You will get one permanent unlock token as well to use on any one of the 55 level weapons or perks.....
The first 5 levels after prestiging, you will have to use the premade weapon class's until Select a class opens up again at level 5. As you level up again all of the standard weapons and perks unlock as you progress.