What about the DCs?
If your invocation shape allows for a DC, calculate it based off the shape. If the essence allows a DC, calculate it based off the essence. If you use a shape and an essence, the enemy has to make 2 saves.
Actually, eldritch blast is always a level 1 spell equivalent, which can be trouble if there's spells like Globe of Invulnerability up. Each essence and blast shape has a spell level listed, though, and the EB gains that spell level (or the higher of the two, if applying both types to one EB). So the essences and blast shapes are also useful for the increased spell level.