Let's say that you're using the Advanced Search to find yourself a good manga to read this day. You go down to where you can select the category the manga falls under and you notice that you can only add categories, but not blacklist categories. I suggest that we add two boxes - one in green and one in red - where the red box displays categories that you do not want to see, and the green box displays categories that you want to see. This would be useful for the people who are looking for a type of manga that appears to be seen a lot, but you have some categories that you don't like to see.

Tell me what you think about this idea

You can already do this: by putting the symbol "-" in front of the category you write in the box, manga with that category will be excluded from the search.

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My current top 3:
Fullmetal Alchemist
Pandora Hearts
3-gatsu no Lion