It's possible that the black fabric dry rotted over the 20 years it sat waiting for use. that would explain the shredding. That's what happened with a cotton drapery that I'd had up for about 20 years - never washed in all that time (shame, shame!). Finally took them down and washed them and they fell apart, huge tears. Oh, well.
I can offer no explanation for the sooty stuff. Yuk.