Help! Is this Fungus or Mold on my plant? (pics)

Hi
Let me start by saying this is my first time growing so my experience and knowledge is extremely limited. With that out of the way, HELP ME! I planted 10 random seeds I collected from various bags of herb I've gotten over the past year or so. Things started off pretty good, Seven of the Ten grew, and Six of those were Females. They're growing in pots in my backyard.

On to the problem...
I went away over the weekend and I when I came back one of the plants has some very dark brownish-purple stuff (either on the surface, or colouring it from the inside) on some of the leaves localized around one of the nodes I think.
I'm worried that its fungus, mold, or something else that may threaten the health of the rest of the plant(s).
I took some pictures and I was hoping someone a little more knowledgable could help me out with identifying what it is, and if it is a problem, what I could do to minimize the impact.

Heres a shot showing some of the presumably healthy plant around the problem area, and a magnification of the part in question:



And here is a close up:




So, any ideas or advice for a concerned Green Thumb?


Thanks,
Mr.NiceCream

some sort of bug man get some neem oil on as soon as possabile.

looks very odd. not sure what that is.

i get that sometimes on my clones but mine doesn't have that dark of a color like yours. Few days or wk from now that spot will deterreate.

Looks like a bird flew over and let loose a bomb. (lol) Seriously, does it come off if you rub it? You might try washing it off if it does rub off. If you can't get it ID'd in the next 24, I'd cut it off at the node below and then just watch. GL HH

My little one had the same thing everytime it grew a new node it had some weird looking brownish black crap on it. some1 told me it was mold/rot so i topped the head off my plant where the crap was on the leaves. it grew back 2 heads with the same color on the plant and even the new leaves coming out of the stem had it on it but after 3-5 days when the leaves got big it went away..... i have no idea what it is.... luckyly mine comes and goes after each new node

oh and what type of plant are you growing? is it possibly some seeds from some ragweed out of a "nick/dime" bag?