This sounds like slime mold. It's not uncommon on lawns, particularly in wet conditions. It does not actually harm the lawn, and will disappear if conditions dry out. Slime is kind of neat -- it actually moves around at night and in early, damp mornings. Then when it gets hot, it stops and just looks like this dark patch on your lawn.
You didn't specify if the lawn is dying out underneath these patches. If it is, then it's NOT slime mold and you should take a sample into a garden center or extension office for an ID.
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