Mindwipe said:
It would make it easier for the people who are requesting manips, but not anyone else.
This is it exactly. As cool as it would be to have a browseable manip request database and all, it wouldn't work too well on here. Every blacklisted image reduces the total visible posts on a given page by 1. If we allowed this, you can't honestly tell me that most people wouldn't just start uploading their favorite images alongside the tag, knowing that, as a plus, they may even get it manipped. This would result in page after page of either unrelated content or empty pages.
It's not that hard to link to it from somewhere else. You can just as easily find the source a good 80% of the time with saucenao.com and just link that, if you cared enough. So there's no reason to implement a detrimental system such as just allowing them on the booru's post list.