I think they would lose some of the SUPER part. WL are HIGH production and LARGE JUMBO eggs. Other colors are not as high production or as large of an egg..... I know Exchequers are not, pretty sure brown are not.
Not sure on the color you would get but a Lavender is RECESSIVE meaning it will not express the first generation. If you breed those chicks back to the lavender you would then get lavenders. I don't really know the genetics behind a brown leghorn. You would get some colors, Lavs are really a black birds with a diluter gene (Lavender)
The basic answer is YES you would get a blue egg layer. Have you tried it in the chicken calculator? I am not very good with it myself.