Thanks for the question and thanks for using JustAnswer.
This is a fairly common problem and is one of those things that can be considered normal or might be a symptom of something else.
I have a cat that does this as she's gotten older. If you notice when cats age they lose some of the fat that is situated behind the eyeballs and keeps them pushed forward. It is a normal aging change for the eyes to sit a little farther back into the orbits (sockets). In my cat - and other older cats I've seen - it appears that this gaping of the soft pink tissue in the eye causes an accumulation of eye secretions. In this case, of course, it's no big deal and wiping it out is the best treatment.
Also in cats there is a duct that runs from the corner of the eyes into the nasal passages which is used to recycle or dispose of tears. If those ducts are plugged or not efficient enough some tears will run out the eye and as they accumulate bacteria, dirt, debri they turn black and can look like what you are describing.
In both of the above situations there is no redness to the eyes, no squinting, no pain.
If you see puffiness or redness to the tissue around the eye (conjunctiva) then this is probably not normal. It would be best to see the vet so the appropriate eye drop or eye ointment can be prescribed.
I hope this makes clear what I'm trying to tell you. If I have not answered your question I will be happy to follow up with you.