Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting for a response, but your requested expert isn't online which delayed your question coming up on the list for all to answer. I would like to help if you are still interested in an opinion.
I am very concerned about Dr. Shel***** ***** (which is a great name for a ferret).
Males have a very small diameter urethra and as such if urinary calculi (stones) form and they try to pass through the urethra they may become lodged. As you can imagine this is very painful for your fellow, and should the urethra become completely blocked the back pressure on his kidneys
can cause kidney failure.
The dark color to his urine can be due to blood in his urine and/or metabolic wastes due to organ disease which led to the formation of the calculi in the first place.
In your fellow's case it sounds like some urine is being passed, which is good, but that he is having to work very hard to do so. I highly recommend that your boy see a veterinarian very familiar with ferrets as soon as possible.
If you would like to read more about urinary tract calculi here is a link to an article:
Best of luck with your boy, please let me know if you have any further questions.