Simply means "inter-racial sex". Can be used as "mud-larking" (hyphenated) or "mudlarking". It is non-derogatory and non-condemnatory. It applies equally to either female or male pursuits. Often it is used when referring to "exploratory" experiences.
"Well, I think she was just in for the mudlark"...
or
"Guess who's been mudlarking just of late?"
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