IF the bilge had that much water in it, and the boat was out of the water, with dehumidifier cans in it... it has some serious deck leaks by the sounds of it. Serious deck leaks usually lead to core delamination problems, if the boat has a cored deck.
The blackish water is probably due to mold/mildew growth.
Most well-found, well-kept boats are going to be pretty dry, especially if they're kept on the hard and if they've got dehumidifiers aboard. The fact that this one was on the hard, and still had a lot of moisture on the inside speaks of an underlying problem IMHO. My boat is usually so dry that I can see the dust floating in the bilge..
All of the water that I've gotten in my bilge was captain induced...
Mold and mildew, once they get a foothold are exceptionally hard to get rid of. It is probably all throughout the bilge, and in all the nooks and crannies on the boat... many of which you'll have no way of cleaning. If the boat has a hull liner, the space between the hull and the liner will be covered.
If there isn't a deck leak, the other source of water could be saturated laminate... If the boat is heating up during the day—solar heat does a lot there, the air will tend to pull the moisture out of the hull and anywhere else that is saturated,... then when the hull and boat cool off at night, it will condense out and drip along the interior of the hull. That is probably how the fabric and cushions got damp, and the cause of the mildew/mold.
I would recommend bailing on it...there are plenty of boats out there that are going to be in much better shape.