Opera Blink refers to the Opera browser from (desktop) version 15 onwards, which uses the Blink rendering engine (a fork of WebKit).
In February 2013, Opera Software ASA announced that their products would transition to using Chromium/WebKit. It was later announced they would actually switch to Blink, the same fork of WebKit that Chrome now uses. Prior to this, they had been using the Presto rendering engine.
The first Blink-based version of Opera was Opera 15, released as a successor to Opera 12 at the end of May 2013.
This tag, opera-blink, should be used for questions that are specific to the newer Opera powered by the Blink rendering engine; questions specific to the older version should use opera-presto, while questions not specific to either should use the generic opera tag.