Just got my first slant six

IIRC, the oil pressure light comes on at 6 or 8 psi.... it is clearly low, indicating a worn pump and/or crank/rod bearings. I would put in a fresh oil change with 10W40 oil and see where it ends up. Is there any odor of gasoline in the oil?

If your readings are indeed that low, then the cylinders and/or rings are all well worn; you will typically find around 125 psi new. But, they are all quite consistent and that would be unusual to all be that low AND that consistent....I suspect a test issue first. Soooo.....
- Did you use a thread-in type of compression gauge or one you push on the spark plug holes? If the push-on type, borrow a thread-in type and re-test. And, borrow another anyway as yours just may be inaccurate.
- Also, prop the carb wide open and keep it there for the test.
- Crank the engine over at least 4-5 times per cylinder and then take a reading
There are a few other possibilities for low readings like stuck rings, poor valve seating or a umped timing chain, but first make sure the readings are correct.

The black spot under the exhaust is likely just incompletely burned fuel or carbon in the exhaust pipe. The cold start idle could be too rich or the ignition not strong (and not completely combusting the cold mixture). That issue may just be due to tuning, choke settings, etc. I would not be discouraged over that alone.

The plugs do not look all that abnormal for old plugs. Put in some new ones. There are oil seal rings around the upper ends of the plugs cups and they rot and seep oil; a new set can be had for a few $$ from NAPA.

How did the engine run in general?

BTW, it is very likely overdue to have the valve lash set and the timing set. Start reading up on that.