One of Trillie's nostrils looks black (for some time now). His behavior / appetite are normal.
What could this be?

It looks to me like maybe he got some dirt in it. It doesn't look like an injury. If it's dirt it should disappear eventually.

thanks Carolyn,
I hope it's just dirt but my association is that it rots.. can it be something fungal?
It also spread above the nostril a little bit. On the pictures where he is looking straight and both nostrils are seen, this nostril really stands out by being black..
If it's dirt, would it help to gently clean it with wet cotton swab? can I try any over the counter meds on this area? One of my rabbits supposedly has cancer (not confirmed), my expenses on her care are high recently (speaking about possible vet visit with Trillie).

You could try washing it gently, but obviously you don't want to get a lot of water in his nose. Or you could just leave it alone and keep an eye on it to make sure it doesn't get worse. If you can't wash it off, then the only way to be sure of what's going on is to see the vet, and it sounds like you already have a lot of vet bills right now.

I took a better look at this area. It doesn't seem that the nostril itself is a problem but I have noticed that just above, where feathers start growing and the cere ends, the skin is cracked or fractured... not sure how deep it goes. Do you think I should apply medication on this area to make sure it's not getting infected?

I would tend to leave it alone and let it heal by itself. If you do use some medication, make sure that it's not something greasy that could get into the nostril or get rubbed off on the feathers.

So, was it a little wound? Did it heal up, or did he finally get around to taking that long overdue shower?

I don't know. It looks about the same but he behaves normal. He doesn't seem sick, weakened or depressed.
as I already mentioned, it looks like he cracked the area where cere is connected to his skin. It's just above the nostil, it's seen on photos as a thin black line
Trillie damages his beak rather frequently bumping into things when they all fly. It always healed in the past.