Black oil, low power, 6.5liter 1995 GMC problem

Hello,
This is my first post. What a great site!
I very recently bought my first diesel, a 1995 GMC 3500HD 6.5liter turbo flat bed. I know little about diesels, so I have a few questions.
I changed the oil after having it about a week. The old oil was as black,
as coal, and stained my hands like it had carbon in it.
I checked the oil after probably 1000 miles and it was three quarts low, with no leak being noticed.
Other conditions are:
The power output is low.
The vacuum pump locked up after I had driven it about a week.
The check engine light comes on often for about five seconds.
The fuel mileage is terrible.
Starts fine.
Mileage is 170k miles.
Oil pressure is 40 psi warm running, 20 psi warm idle.
Did I buy a dog?
Thanks for any help you experienced fellows may have.
Charles