Priest talent · row 1 of Shadow, from level 10 (5 with Talented 5/5) · Physical

Blackout Rank 5

BlackoutRank 5

Gives your Shadow damage spells a 2/4/6/8/10% chance to stun the target for 3 sec.

Game files · build 1.60.1.70245Read from the WoW Forever beta client, build 1.60.1.70245.

Compared with Classic

Classic tooltip

Gives your Shadow damage spells a 10% chance to stun the target for 3 sec.

Data	Classic	Forever
effect 1 value	0	10

What Blizzard said about Blackout, each line checked against the files.

Matches the files
Improved Psychic Scream: Blackout is now a pre-requisite.
The files Talent edge 136300 runs from Blackout to Improved Psychic Scream; Classic had no prerequisite.
Improved Psychic Scream Blackout Class deep dives: Priest and Warrior

69876 Sep 16First beta build we archived. No changes in the client files since.