In a pedantic sense, using the Warlock's essence and shape invocations as intended goes against the RAW. Invocations are spell-like abilities, and all spell-like abilities take a standard action to activate unless otherwise noted. I don't think the essence/shape invocations explicitly say that they're used as part of another action... thus reading it strictly implies that you use these invocations as a standard action, and then all subsequent uses of eldritch blast are modified by their stated effects. In this case, you would need a duration.
Richard Baker cleared up how they're intended to work, however.