I think it is old blood. Some women get this right before their regular period comes on.
Please no more unprotected penis in the 'gina. You really have no business risking pregnancy.
So, does it smell? I'm asking because blackish discharge with an odor could be . . . don't get grossed out because it DOES happen . . . but a tampon forgotten and left in can cause that. It is almost always accompanied by a strong and foul odor though. Otherwise, brown spotting and I guess brown spotting that looks black can happen due to hormones and is old blood. Especially common if you are irregular as you describe yourself, so it could be that.
However, you are having unprotected sex. Even if he doesn't fully ejaculate inside, an unprotected penis without you being on the pill is risking pregnancy. The penis emits small amounts of sperm at all times before ejaculation. That's enough to make you pregnant, unfortunately. If you do not get your period, you should take a pregnancy test at 2.5 weeks after the unprotected sex. And consider birth control if you plan to keep doing this.
I doubt you are pregnant but you'll need to rule it out. good luck