Im not expert on cat colours and which genes carry which colour / which dominance.
I will take it purely theoretically.
Assume white gene is recessive.
So a grey mom (or blue as grey is often called for blue) and a black dad can both have a recessive white gene, but it will not show as long as they have also another, not white gen "stronger" than white, dominant over white.
If both mom and dad thus have this recessive white gen, there will be about 25% chance they will get kitten with both genes recessive white; ie white kitten.
In practice, about 1 kitten in a small litter, about 2 kittens in a big litter.
Half of the siblings will have the usual colours but be carriers of the white gene.
And the last 25% will have usual colours and dont have any recessive white gene.