What distant said is true ^^

I actually do use black for lineart but most of the time, after I've coloured, I colour the lineart as well with a darker version of the colour it borders with - as a general rule for colouring though, I NEVER use #0000 black in my art either c:

Also it's worth considering that longer strokes of the pen will result in less shaky lines! There are even brushes with stabilizing in them in most art programs :D

But you're well on your way, and yes, do open up a shop! Commissions are GREAT for improving :3