...wouldn't they both be blue velvet shrimp, especially if they bred true blue?
You are ignoring the fact that 99.9% of blue velvet's red markings fade. It is part of the mutation.
The blue rili keeps any red markings it may have.
As to whether it can be categorized as blue rilis, although the phenotype looks the same, the genotypes are different.
Do they both look blue. yes.