Need Help Roland SP-300i cyan not printing

It sounds like the cyan siphoned up some of the black ink. this happens to me on my 540 w/ the Yellow. Try doing several clean cycles followed by a test print after each so you can see if there is any improvement. Once I get some cyan showing I'll print a solid block (full width) of cyan until it looks normal.
Note: When I say test print, I mean the button on the front, not the image you shared.

 

Run this test print, take a picture, and post it here please. Thanks!

 

This will happen when 2 conditions exist:
1. Old captop or something on the captop which allows the 2 channels to siphon through the cap top
and
2. The level of ink the the black cartridge is substantially higher than the level of ink in the cyan cartridge. - it could go the other way if the ink levels are reversed or it can happen on the Magenta/Yellow head the same way.

Hopefully the contamination has not reached all the way to the ink cartridge.

You need to replace the cap top - you should probably do both of them at the same time. Then work ink through the cyan channel, either by cleanings or by printing a large cyan block on some cheap media. (Or a combination of the two) Once the cyan block starts to come out cyan rather than the dark color you described, you're good to go.

Good Luck,

 

All of the above, plus check that your printer is level.

I have heard this can happen when one of the carts in full and the other almost empty.

A hair or fiber on the head can cause the ink to wick out one head and up the other.