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As in dribs and drabs. Can it be used in the singular, I wonder and can it be used without its partner drabs?

On the other hand, you can use drab, in the non-blah sense, without a drib in front of it.

I'd guess this is related to dribble, though. That would make drab rhyming reduplication.

If business is really, really bad, I should imagine one could say the customers came in so slowly it was just a drib here and a drab there.

Excellent use of the singular, Slava. And as you say, business is REALLY slow.