Help!!! My Dwarf Gourami Is Turning Black

Help!! I have a dwarf gourami who's face, breast, bottom in are turning a grayish black color. He is hanging out above the heater toward the top of the tank. Currently in a 30 gallon with 1 other dwarf gourami, 7 neon tetras, 1 bristlenose plecos, and 4 black fin tetras. Water tests show 0 nitrate, 0 nitrite, pH is 7, kh is 60, Gh is 80. Ammonia is testing good too. I've heard that he could just be maturing. But he doesn't seem very active and when I made him move he goes right back to his corner. He fed well last night and this morning. I've had him for about 5 weeks now. Any ideas on what's going on???

 

I read some threads and they say that there's turned black because of stress

 

2 male dwarf gourami will usually fight. I have 1 male dwarf and 1 honey dwarf. Had to put honey gourami in QT because dwarf gourami was stressing him out and he became bloated and turned grey where you described. Fed him peas cooked and skinned soaked in garlic. Garliccsuppose to help them fight off disease. In a week or so his color returned and I put him back in main tank. He went after my oto in QT so out he went. He grew a pair as he goes after dwarf gourami at times and they are starting to coexist.

 

I've had both males in the same tank for 5 weeks now. And they've never fought. I am usually home and watch them throughout the day and have never seen the remotely go after each other. In fact they are usually found swimming next to each other. So when I seen the one alone and hiding toward the top it was alarming. I will quarantine him just in case for a few days and seeing he improves. Any other suggestions are greatly appreciated

 

That is cause it gets in mating mood !!!
Males get a black blue head / breast.
Be aware it also could change in a killer at that moment !