The black goo is rubber (or synthetic rubber) breaking down.
I use elastic when I make anything requiring some stretch, elastic is thin strips of rubber(/synthetic rubber) running parallel with each other and all stitched together. As an alternative you could use a single wide strip of rubber (/synthetic rubber) but it is more vulnerable to heat and chemicals, washing in bleach then drying in a hot spin drier is a killer.
If you really value this suit you may have to unpick all the waist stitching and put in new elastic, then resew it. This could take some some and spoil the looks of the suit. Shopping for new swimwear can be exciting!
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