If you got a splash from two "blacks," then they are both actually very dark blue. It is not uncommon to have blues so dark that they look black. However, compared with a true black in good light, you can see the difference. If you have a good blue, a black-looking-blue and a black all side by side, the black-looking-blue actually does look closer in colour to black.
Genetically, however, they are blue, which is incompletely dominant--1 copy dilutes black pigment to blue; 2 copies further dilutes the black pigment to a light slaty base with dark random, irregular splashes throughout
Lavender is an entirely separate, recessive gene that dilutes both black and red pigment. To have any affect on appearance a bird must have two copies. Becasue they are unrelated genes, a bird can be both blue (or splash) and lavender.