Extend spell like ability mentions:
The creature can use that ability as an extended spell-like ability three times per day (or less, if the ability normally usable only once or twice per day). When a creature uses an extended spell-like ability, its duration is doubled. Spell-like abilities whose durations are instantaneous, concentration, or permanent are not affected. The creature can only select a spell-like ability duplicating a spell with a level less than or equal to half its caster level (round down) -1.
Based on this, can a warlock apply this feat on any of its essences such as Brimstone Blast or Beshadowed Blast to increase the duration of the effect?
Example: Applying Extend on Beshadowed Blast would case victim to be blinded for two rounds instead of one. With Brimstone Blast, it is calculated by 1 round per five class levels you have, so normally for a level 10 warlock target would burn for two rounds but with extend, four.