When activated, increases the critical strike chance of your next offensive ability by 100%.
Cold Blood is available as a talent in the Assassination tree. It increases the critical strike chance of the next offensive ability by 100%.
"Next instant special attack" may be a more accurate way to reflect the abilities that Cold Blood will modify than "next offensive ability." For instance, Cold Blood will NOT affect damage-over-time attacks like Garrote or Rupture, even with the 4-piece Tier 8 armor bonus which allows Rupture ticks to be crits.
It is still possible for the Cold Blood-modified attack to be dodged, parried, blocked, or miss, due to the table-based attack resolution system that WoW uses; in this situation the Cold Blood buff (along with the 100% crit modifier) will not be lost, and will still apply to the next special attack.