I bought a Pixma ip8750 to try and make my first steps in printing. It does print in hexachromy, so it has a grey, a dye black and a pigmented black. However the results I am getting in b&w prints are far from good. Just the glossy photo paper had a decent result, while matte coated paper (300gr) were greenish or with a touch of magenta, depending on if I choose common paper or matte photo paper in the printing menu. Could this depend just on the paper type? Just glossy paper has good blacks, or am I doing something wrong?
The only suggestion I can give is to go to your printer options in your operating system and experiment with the different paper stock options. Also experiment with the color calibration in the printer options if they are available to you.
I'm honestly not surprised that only glossy paper had decent results.
I'm making a big assumption here from looking at Epson's website for this printer that when they claim it is "Ideal for photo enthusiasts" that you should be using glossy or photo paper for high quality print. Your matte coated paper might just not be good enough.
From personal experience with various printers I find that the choice of paper stock makes a huge difference with the correct settings.
You basically need to experiment a lot because every printer is different and it has in the past taken some experimenting to get closer to the results I need with a new printer.
Two things I would have tried:
First: I recommend to convert black and white photos to greyscale. That way you avoid a color tint, but on the other hand the result might not be as dark as you want to. I don't know how your printer works, but with the additional black and grey colors, I'm guessing there is a possibility to use them in a greyscale scenario to get a satisfying result.
Second: Adjust the color of the image so that the darkest part is close to CMYK 30 20 20 100 (more Cyan than Magenta and Yellow to avoid a brown tint) do a test print and adjust CMY from there until the darkness and tint is to your liking. It can be hard to adjust that in a photograph, so another solution can be to adjust the levels in your printer, either via a control panel or in the driver settings. Again, I'm not familiar with the printer, so consult your manual.
The color of the paper will affect the color. Coated paper (gloss) will give you a darker and clearer result because the colors are not absorbed, but stays on the outside and the paper reflects more light. Uncoated will absorb more color and the print looks less saturated, or faded, in lack of a better term. A similar effect can also be seen on coated matte paper.
Do not use registration black for anything else than registration marks. Anything above 300 % cover is not recommended, it will "never" dry and you might get smudges or a metallic effect on the darkest parts.
First make sure you're outputting the darkest black possible.
From the link above I would try Registration black which is 100% of all the colors.
Secondly yes paper coating makes a big difference in appearance. Glossy photo paper has a clay coating that can absorb a lot more ink than plain paper. With that print setting the printer puts out more black and the medium absorbs more black.
Any Paper besides glossy photo paper makes photo prints look not as good and lighter. Medium gloss, "frost" "Lustre" or "semi-gloss" can produce good results at less cost.