Definitely looks like an ink flow issue.
The ink system operates like a siphon, who's flow is refreshed by the ink pump when the printheads park on the capping station.
Black shares an ink pump with an adjacent color, i think magenta. So you can most likely rule that out, if the magenta is printing 100%, unless the black waste line is cracked at the pump. Removal of back-cover of printer allows access to ink pumps, above your waste container. Worth the time to gain access & observe the waste lines while running disway wash cycles and pouring cleaning solution down the capping station.
You can also rule out data issues, by performing a "data swap". Diagram by artbot available for search on these forums, where you basically swap the magenta ribbon cable with the black cable and confirm whether the issue stays with the printhead.
after that, my best guess would be that your problem is still with the ink flow. Chances are that it is either an improper connection between the printhead & captop during the refresh cycle, an air bubble in your ink line, or you have a cracked printhead manifold (black plastic tip on the printhead that the ink-line/damper sleeves onto.)
solutions:
capping station: if you apply a slight upward pressure to the black seal/gasket that the black printhead parks onto the capping station, you may be able to lift it slightly which will allow a tighter "fit" when the printhead parks back onto it.
air bubble in ink line: run a bunch of fillupink cycles and observe the ink lines for an air bubble. if there is one, keep running the cycle until it makes its way to the damper.
cracked manifold: last resort, but is pretty common for people that have had issues after recently fiddling with their printheads/dampers. It is an inexpensive part you can order on ebay, or supplier, or salvage from an old printhead BUT replacement is a pain in the butt, and could cause more problems if done improperly/unnecessarily. Requires removal/reinstall of printhead & alignment procedures.