After further research, I would say this is a DM's call. Using an Invocation usually provokes AoO, and while the reading of the invocation doesn't remove that limitation, it almost implies that there is no AoO.
Might put a post up in the Rules forum and see what the learned sages there have to say.
Hideous Blow(CArc p134)
<Invocation(blast shape)>
– As a Standard Action, make one melee attack.
If the hit is successful (even if it does not
damage), then treat the opponent as if he/she
were also hit with the invoker’s Eldritch Blast
(including any special effects due to Eldritch
Essence invocations).
Effective Spell Level: 1st.