Power issues Skilsaw Model 77
A properly running brushed motor should have a chocolate brown appearance this is a oxide that forms from moisture and brush wear. I do extensive work on brush based motors there is always arcing but it should be uniform and not very large. If the saw was in a damp location the windings may be saturated causing the higher than normal arcing. Using an ohm meter on the power plug with the trigger pulled you should see a low value of resistance, rotate the blade a little at a time you should have a similar resistance value through a full 360 reveloition of the commuminator. If you don't have any dead spots and the saw was in a damp location you may be able to bake it out in a oven at approx 150 deg F. I have to do this to motors that have been sitting on the shelf in an unheated warehouse when they are an open drip frame style.
If you have a electrician friend ask to meg the motor, this is a high voltage ohm meter that checks for leakage in the windings to the frame while turning to the different field windings with the trigger pulled these values should be at least 10meg at 250 v higher is better.
Checking the continuity blade to blade on the plug with the trigger pulled is usually less than 10 ohms and on some motors less than 1 ohm. If no continuity verify there is nothing stuck under the brushes and they are making contact, I have never seen a brush base motor With more than a couple fried coils if no continuity through 360 degrees rotation of the com a brush contact wire may have burned off or the wire at the switch since you said there was voltage at the switch.