Yup, turns out it's a HUGE game bug. Something I hope BioWare will look into. If I'm wrong, then I do apologize, but it's clear-cut and dry that this is a game bug and I'll tell you why:
I remember now, I was doing the "Justice" quest for Anders and for some reason I didn't know what the heck I was doing... I totally forgot to distract the Grand Cleric (had to start a dialogue with her), and Anders will appear by your side, etc. So I didn't do this the first time around, and that's when I encountered the bug.
I entered the Chantry, and didn't know what I was doing, and then I exited the Chantry. I went directly to Ander's Clinic and spoke with him. He responded exactly to me the way you would if you completed the quest normally (not like I did by exiting the chantry and not talking to Grand Cleric).
By this time, he is already "one" with the demon spirit inside him and that's why his armor dawns the black color. And he is also in the blue "friend" zone on my playthrough.
So because I messed up the Chantry part, right when you're done speaking with him, to add him to your party (because you should only have 3 people total, after this quest), he is not selectable at all.
I think it's a bug, because the mission switches from a party of 4, to a party of 3 when he leaves you inside the Chantry to do his "thing." And when you return to Ander's Clinic, you are then again left with a party of 3 total. When you go to the "select your party" stand thing, you are suppose to be able to select him. In which my case, I wasn't able to at all.
It's really weird. I don't know how to explain it better. As long as you didn't abruptly leave the Chantry part and you continued on to talk to the Grand Cleric, you should be okay. And he will continue to be selectable as a party member.
I hope this helps. I lost 5 hours of gameplay. I reverted back to an earlier game save which was at around 35 hours. But I'm happy now that I've done it. The way I specced Anders, he's basically, a healer/tank. He was tanking one of the mid-sized dragons in the High Dragon fight, and he would not go down. I was pretty amazed that I actually nailed a spec for him because I usually suck at these things.
Thanks you guys for all your help at such a late time at night. Well, it's actually 1 am here.
Take care!
P.S. BioWare, I hope this helped you guys with bug/patch notes or something. Whatever you guys do with bugs like this.