I've got a problem with masking. I've made an object and masked it over. The masking effect is visible, but it should go behind the object. The transparent part should be at 0% visibility, but somehow it's still visible and I'm not sure why. How do I fully hide this part of the object?
Either your mask opacity is not 100% or the colour you use isn't fully black.
Especially if you work in RGB and you chose the standard black swatch, it is possible that it is instead a very dark gray (actually CMYK black), which translates to barely but still visible in masking.
Change the colour of the masking black object to actual RGB
(0,0,0). If that doesn't work, check your mask opacity.