Black water in toliet

This is pretty gross- My upstairs toliet keeps filling with black water, or rather, its not black water when it fills, but if it sits all day it turns brown/blackish.  I've cleaned it 4 times in 3 days, so its not "dirty"- any ideas?
Thanks!

It's a bit hard to diagnose from afar, but there are actually a few things that could be going on. If you give me a bit more info, I can probably tell you what's wrong. (Growing up I learned quite a bit about plumbing, then did repairs on my own house, then worked for a magazine for builders and other tradespeople.)

Is that the only place you're getting this? Are you getting discoloration anywhere else in your plumbing?

Does the water smell bad or just look bad?

What does the water in the tank look like?

Is the water in the toilet thick or just discoloured?

Hey, thanks for the quick reponse!!

Q: Is that the only place you're getting this? Are you getting discoloration anywhere else in your plumbing? 
A: This is the only place I'm getting this, no other discoloration anywhere else in the house. 

Q: Does the water smell bad or just look bad?
A: Its not a terrible smell, not that I've been putting my nose up close, but it definitely has a sourish odor.

Q: What does the water in the tank look like?
A: No idea, toliet is connected into the wall, I can't see the tank

Q: Is the water in the toilet thick or just discoloured?
A:  Its pretty thick and discoloured

Thanks a bunch!

Sounds like you probably have a backflow problem. Since it's the upstairs toilet, you won't get the full-on raw sewage stench that you'd get on a ground-floor toilet with backflow.

It could be something as simple as something caught in the drain line. A plumber might snake the toilet first to make sure it's clear.

If it's not, it means that you have a seal or pipe somewhere that needs replacing.

I've actually seen this happen with a toilet that was left to dry out. The water in the line starts evaporating and you get some gassing. When you reinstate the toilet, the water gets discoloured for a while until everything's got water where it needs to be.

You're going to have to call a plumber out to really get to the problem. There's no easy solution, unfortunately.

Yeah, all sounds very logical! Thanks for your help. 

Sounds like you probably have a backflow problem.   {.....}  It could be something as simple as something caught in the drain line.

I concur. 

You say it's the upstairs toilet.  Does that imply that you also have one downstairs?   If so then flushing that one if there's a substantial blockage will easily result in backflow.

Other fixtures could also do it, but it depends upon whether you have a single-stack or multiple-stack drainage system and whether sinks, baths, etc. discharge directly into those stacks or over gullies.

Yes, I have a downstairs toilet as well.

I wonder about the other stuff too...

Hmmm

Well, knock wood, the toliet hasn't been too bad the past few days, so we'll see...