The dark comb is often an indicator of cardio/pulmonary issues. Listen carefully to her breathing. Does it crackle like crumpling cellophane?
A dark comb can also indicate multiple organ failure. The yellow in the poop can also support this hypothesis, meaning the liver is failing. Is she drinking a lot of water? That's also a symptom of liver failure.
There is also another possibility - lymphoid leucosis which would be the underlying pathology that would cause all of the above.
I'm sorry. Your hen is very sick, and may be in her final weeks. Supportive care to make her happy and as comfortable as possible is what I suggest, meaning keep doing what you've been doing, but it's not likely she will get better.