Are you sure it is black ABS, and not maybe burned particles of Red ABS?
If it is black ABS, you could try enabling a wipe tower to minimize it, however the oozing of the second (idle) nozzle is a common problem for dual extrusion which may always be visible to some extend.
Lowering the temperature so you have less molten plastic is also a good suggestion.
try lowering the printing temperature
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