Alright, first of all let's just agree on the fact that all-black 109s did exist. And as FalkeEins stated, they were encountered and shot down during daytime too. It clearly happened, let's just accept that first
Here is one of several photos from Luftwaffe Im Focus. This aircraft is all-black.
Clostermann is a story on its own. He describes green&ocre Luftwaffe aircraft with yellow bellies. Personally I think that after staring into a blue sky for an hour, everything else gets yellow thanks to the brain trying to compensate(*). So these aircraft Clostermann describes were just standard 74/75/76 camo machines, with some yellow added by Clostermann's brain
The all-yellow Fw 190 undoubtedly had a different-from-usual paintscheme, but was most likely all blue RLM 76 or perhaps primer-only RLM 02.
So let's just assume for a moment that Lt. Hagen wasn't colour blind and indeed shot down an all-black 109. Do any day losses of (Nachtjagd) units near Stettin match with his claim? I unfortunately have not much information of Luftwaffe losses (besides JG 26), but I hope somebody else on this forum might know more about this.
(*) My bedroom happens to have purple curtains and when waking up the walls in the house look green, until I open those curtains, then they're white again! The brain does funny things!