First when you are doing a test print You are doing it through the menu right? and its showing the lines for each color range? Cyan,magenta ,yellow and black? I do not remember how different the SP300 is from my VS640. I know its pretty similar , but I think it has 2 print heads and my VS has a single head. I think the Sp 300 shares a head for Cyan and black, and then has another head for Yellow and Magenta. When you do the test print through the menu it should be printing each color and use just that color. When you print a file like a design or lettering it mixes all the ink colors to get black . So if you are doing a test print with your own image , you might have a problem with the Magenta/yellow head and that is why the black looks blue. Your not getting the Magenta and yellow that help make a rich black. So if you have not printed the test print through the menu , try that and see what it looks like. It should be a bunch of lines and each color ink will have its own pattern. If you do not have each color coming out see which ones are missing. Hopefully you have some of every color on your test print. Do your crop marks for cut registration print out still?? I had one of the 2 heads fail on my SP300 and it stopped printing the crop marks. The printer makes the crop marks using only black ink. So that can also help you figure out what is the issue.
How often do you manually clean the printer? You could have some gunk on the wipers that is resulting in the heads getting some gunk transferred instead of wiping them. When you do the cleanings through the menu, try not to do the powerful cleanings. The regular cleanings use way way less ink. I was told by my Roland Tech that he powerful cleanings waste a lot of ink , and most of the time if a cleaning will fix it, the regular cleanings will do it and use much less ink. How often do you print? How old is your capping station? If you do not print very often, and or you have not replaced your capping station in the last year or so ( again depending on how much you print you would need to replace it more or less often) you may want to try and fill the caption station with cleaning fluid and park the print head overnight. You should be able to find instructions to do it by searching SP300 overnight soak. If you do not print often , you could have some ink clogging the print head that the overnight soak could clean out. Also check and make sure you can see ink in the lines. You could have an air bubble in the ink line.
I would start out with doing a test print from the menu, then depending on the results, do a manual cleaning and then let the heads soak overnight or for 24 hours and then run a regular cleaning and do another test print again using the menu to print the test pattern. While doing the manual cleaning, look for anything that is gunked up, check the ink lines to make sure they are a constant color all the way to the head where you can see them. If you see air in the lines , I think that could be the sign of bad dampers, Its been a longtime since I ran my SP300 and I did not work on it myself as much as I have on my VS640, so I'm not sure how different they are. I would try searching this forum , I found info on this topic many years ago when I had issues with my Sp300. THere was a Youtube video I watched back then on how to manually clean a Roland SP300 or SP540 . the 540 is the same , just wider. But the menu and steps for cleaning would be the same. So include SP540 in your search. the 300V is the same printer , but different firmware and runs on Versaworks instead of Colorip. But as far as the printers and cleaning and the parts , they are very similar if not identical.
Good luck!