Maybe a minor air leak in the intake system on the leaner side? The
intake gaskets have a straight side & I've seen them suck in. Maybe a
loose hose clamp?

A leaking primer is also a possibility - as I recall they usually only
prime on one side.

A test would be to apprach under power, land and check immediately,
avoiding any taxi at low power & see if it is fouling at cruise. I'd
be concerned if the black stuff is generated on one side at cruise.