I may be wrong, but I believe you damaged your Magenta head pushing/puling too much through them when you had the last problem, now you're getting overspray on that channel. I think that issue was a bad pump or cap seal; these printers don't have pressure in the lines, just suction from the pumps.
The reason it appears on "black" squares and not "green" is because the black is actually a "rich black" which includes magenta (and all colors) in its color, whereas the green is only yellow and cyan. I haven't used VersaWorks in years, but can you look at the color breakdown in the RIP? It should show you that the black area is actually made up of all colors in this case.