The mushrooms have stood for too long, allowing them to drop their spores. That's the black thing you see. Can't do any harm, but is not desirable because it makes them a little less potent. When they are big, but still have dense caps, keep a really close eye on them. Better to pick them when they are all still closed than to leave them standing too long.
For the kit, it does not matter much and you can just go on with the next flush.
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