Those are two separate colors. Blue ameraucanas get their color from the blue dilution gene, and that means they also don't breed true. If you breed two blue ameraucanas together, half the offspring will be blue, but 25% will be black and another 25% will be splash, because of how the blue dilution gene works.
To get blue, you must already have a bird that is blue, or splash, splash being two copies of the blue gene. You can breed a splash bird to a black bird to get all blue chicks.
A good blue will have nice darker blue lacing around each feather, like this:
Lavender, meanwhile, is a whole other color gene. It is recessive, so the only way to get lavender is to breed two lavender birds together, or two birds that carry lavender. Often they will be bred with blacks to keep the color looking good, since breeding lavender to lavender over and over and over can cause problems with the color over time.
Lavender is also called self blue. It does not have the lacing around each feather like blue does. This is a lavender:
