One common theme is secrecy. Since the love is illicit, the characters often have to keep it hidden from others. Another theme is the struggle against society. The lovers usually face opposition from their families or the social norms around them.
One 'illicit love web novel' could be 'Gone with the Wind' to some extent. Scarlett's love for Ashley while being married to Rhett Butler has elements of this. Another might be 'Wuthering Heights', where Catherine and Heathcliff's love defies social norms.
No. Writing a blog with illicit sex stories is unethical. Such content involves violating moral and often legal boundaries. It promotes inappropriate and unwholesome behavior, and can have a negative impact on the readers, especially those who are vulnerable like minors.
One option could be certain file-sharing websites, but be careful as not all of them are legal or safe. Another possibility is to search on dedicated manga download platforms, but make sure they are authorized to distribute the content.