From WoWWiki

Blackout
Gives your Shadow damage spells a X% chance to stun the target for 3 sec.
Usable By
Class	Priest
Properties
Location	Shadow, Tier 1
Affects	Shadow spells
Ranks	5
Requirements
Points required	N/A
Related Debuff
Magic Blackout Stunned. Duration: 3 seconds

[edit] Description

Blackout is a Priest talent that has a chance to stun an opponent for 3 seconds. Blackout triggers from damage spells from the Shadow tree, but not from Shadow spells in the Discipline tree (such as Mana Burn or Feedback). It does not trigger off spell ticks (such as each tick of Shadow Word: Pain), only at the initial hit of the spell. The chance to trigger depends on the number of talent points invested, increasing by 2% for each point, up to a maximum chance of 10%. This talent is usually taken by PvP Shadow Priests in lieu of Spirit Tap.

[edit] Rank Table

Rank	Chance
1	2%
2	4%
3	6%
4	8%
5	10%

[edit] Notes

• Blackout can trigger off priest racials like Shadowguard and Touch of Weakness.
• Lasts for 1 second longer than the similar Mage talent Impact.