is it just the one line going to the black? have you reverse flushed the line. occasionally a piece of rubber or something from a cart can get into the piercing needle that can restrict flow. the printer will constantly prime the line, and the cart will pull back on it a bit. you can judge flow by pulling ink on a damper that is flowing... memorize the resistance. then pull ink from the one with air in the lines. i dealing with restriction, you will sense an increase in vacuum force needed and flow into the damper. if so, put an empty clear cart on that line. get some pure solvent (acetone is good), take off the damper and fit the syringe directly to the ink line. push and pull the acetone inside of the line. you will hear bubbles as you reverse the flow.
after this flush that out with cleaning fluid. pull cleaning fluid into the damper. get a read on resistance. if you had a bit of a coagulated clog, or fleck of something you will immediately feel the difference.