These are "free" infusions
Note that the wording on Elemental Blade does not state that the Infusions are replaced. You simply gain these extra infusions. The Infusion ability is merely changed, and you are replacing other abilities to gain them:
This ability alters kinetic blast and infusions, and replaces metakinesis and metakinetic master.
For further evidence that the infusions are not replaced by Elemental Blade, look at Knight’s Resolve:
This ability replaces the kineticist infusion gained at 3rd level and supercharge.
You can see that by taking this archetype you are only losing the 3rd level infusion. If Elemental Blade would already replace all the other infusions, this line from Knight's Resolve would make no sense.