@ConnieinSeattle,
@katev and many, many others here are much more intense rehabbers than I would even attempt, so I'm always happy to see their before and after adventures.
That said, I can share how I straighten straps. If the strap can be removed from the bag, I do that but still dunk it with the bag. I use the same process as
@ConnieinSeattle (except I use towels vs bubble wrap) explained but while still damp, place a towel, cotton tee or similar on each side of the kink in the strap. Weigh it down with a gallon jug of vinegar, cast iron kettleball (because I don't use it for exercise

), or anything else heavy that will flatten the strap. No books as they will absorb the moisture from the strap and towels. Let it completely dry and you should have flat straps. If you have a strap with a very stubborn kink, dampen it and repeat the weight process.
I've not used leather dye to refresh any of my black bags, but many times I apply a very light coat of black Meltonian Shoe Cream to the corners if necessary.
Black, in my opinion, is the very easiest color to rehab. Have fun and Good luck!