Dude, don't mess around with mixing inks. Just completely flush the system with a printer cleaning solution (not sublimation cleaner, that's different stuff!) before switching to the new inks. It's the only way to be sure you don't get weird color shifts or clogs. A few cycles should do it.
I have the ET-8550 and while it's great for color, the black and white isn't quite as deep as some dedicated B&W printers. It's good, don't get me wrong, but if truly deep blacks are your top priority, you might want to look elsewhere.