If that is a single black head with one side blocked a bit you could try this but be careful.
With a clean swab and cleaning solution push the swab up under the head on the side of the blockage. Do not rub the head only apply slight pressure up. This will force some cleaning solution up into the head in the area of the blockage. Most times when my unit has sat and has a blockage that has not cleared up by a couple cleanings, this works.
Never rub the heads as that will cause damage.