i know it will reduce the HP by 1\16 at a turn but is there any use to that ?
No, there isn't, unless you Trick or Switcheroo it away on a non-poison Pokemon. Black Sludge is to heal poison types only, so trickers get a bad item from tricking.
Hope This Helped :D
You can utilize trick/switcheroo onto a non poison type enemy. Otherwise, not so much use
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