Lavender is a Gene(lav) Self blue is a phenotype(a completely blue bird) caused by many genes working together lavender is one of themAlso, it is the lavender gene that causes this coloration - not the self-blue gene.
there are other genes that can give you a self grey/blue looking bird that breed true...Where did you get the information concerning the etymology of the phrase Self Blue? My understanding of the term is that it is used to differentiate between blue birds that breed true & those that don't.