Once you install a content blocker, go to Settings > Safari and scroll down to Content Blockers. Flip the switch to enable/disable your content blocker(s) of choice.
1Blocker, from Salavat Khanov
Adamant, from Cory Bohon
Blockr, from Tim Poller & Arno Appenzeller.
Crystal, from Murphy Apps. (Reviewed)
Freedom, from Zach Simone.
Purify, from Charismatic.
Silentium, from the Silentium team.