Hi mantaskava,

re:
> Is it some kind of shading error or? - https://i.imgur.com/qzNySew.jpg

It's a display artifact just in the realtime viewport display, it's due to the display mesh being too sparse in that area. When you go to export it you shouldn't see it in the export mesh:



For your construction, usually it is good to make things you want to be smooth to extend to be longer surfaces instead of filling in open areas like you've got here. It's difficult to make the fill in piece be smooth to its neighbors. You might have some trouble with doing that here though since it shrinks in size quite a bit near your end there. One thing you could try though would be to make a fill in piece then trim away some open space near the top and put in a Blend surface there.

It may be hard to get very good shaping with this area swooping upwards while the other side is simultaneously flaring out:



It might help get better shaping if all those profiles were further extended downwards so your surface could be more relaxed and then cut it off at an angle rather than trying to have a surface that directly hugs along those contours.

- Michael