I’m not sure if there should be special moderators behind such an app. I think maintaining such lists as a community is possible, like Wikipedia is maintained. I also imagine such an app would use categories of content so people can use it as a customizable filter.
The app could even use it’s own reputation system so “false flags” are punished by a reputation loss, while properly flagging content increases reputation and gives more “weight” to future flags, or perhaps even give more share in the rewards from the network and/or donations. Whether something is correctly flagged would be up to the eventual majority vote. I think that would work. For example, I think most people against “child porn” would not want to flag it under “drugs” or visa versa.