Blacklist is a list of websites or a list of inventory sources that advertisers use to not target. It is exactly the opposite of a Whitelist.
With a Blacklist you run a campaign everywhere except in the placements listed in a list.
In a Whitelist you will only bid on the placements inside the list.
Usually Whitelists are more narrow and Blacklists more broad. And advertisers only use one or another. Most of the dsp’s only have blacklists, as the Whitelists are normally the target.
Main reasons to exclude a placement (i.e. to include the placement in the blacklist) is the CTR and CPA or no ROI.