The blue gene has two alleles: Bl for blue and bl for not-blue. Every chicken has two "blue" genes. Each of these genes can carry either allele. A bird who is "blue" has the genotype of Blbl, or one copy of blue and one of not-blue. A bird who is splash is BlBl, two copies of blue. And a bird who is black is blbl, two copies of not-blue.
An animal can only contribute the genes it has to its offspring, so a black can only contribute the not-blue allele.
That said, some blues are so dark that it is dificult to distinguish that they are genetically blue from their plumage; however in good natural light they do not look to have good black plumage--the colour just isn't quite there--they aren't black enough.