Is it possible to redirect blocked domain to custom page, show custom image, or single color so instead showing error 404 for example on youtube ad space, i can show image or single color?
Yes and no. You can change the blocking mode and return a custom page but it will only be shown when the whole page is blocked. It will not show in a frame on a web page and it will not work when the site is using https/TLS due to security features.