No worries on the black stuff on the trunks, I think it may just be dead moss or lichens - they probably were green or blue but died from the drought.
I've never been a fan of that black weed block cloth, frankly. I think again it's going to deflect water from the root area of the tree. Just mulch with a thick layer of wood chips and it should take care of most of the weeds pretty well. What you need is lots of water to get to the roots of the tree.
Did you test a twig for signs of life? (since you've been working on it, I guess probably it's still alive?)
