Well, if they had a Self-Blue(what most people mis-label as Lavender) split going on. Which means, they can express Black as their phenotype (what you see) but carry a masked genotype (what you don't see) that will only show up if both parents are a split color gene. It looks either Self Blue or Blue, and the Blue (Andalusian Blue gene) only shows if one or both parents are Blue or Splash.
So, my guess is that it is from parents carrying the Self Blue. Confused yet? LOL
With Andalusian Blue (like I have with my Cochins). Here is what you get when breeding:
Black to Black = all Black offspring
Black to Blue = half Blue, Half Black
Black to Splash = all Blue
Blue to Blue = 25% Black 50% Blue 25% Splash
Blue to Splash = half Blue, half Splash
Splash to Splash = all Splash
But with Black splits to Self Blue, I don't know the numbers for sure, but I believe if you have two parents who are splits, then they should produce 50% Black and 50% Self Blue.
I will be honest, I find the whole "Lavender" craze irritating because people have mis-labeled them. They are pretty, but the only true Lavender colored birds, or that that label is used with are some Game birds. I don't think any other breeds have been accept under that label/variety. But for sure, they are beautiful Silkies in that variety.
The way you can tell the difference too is that the shaft of the feathers is that light blue or dove gray color as well as the barbs of the feathers, whereas with the Andalusian Blue, the feather shaft is Black.
That little cutie likely won't have a big crest. When they hatch they tend to have a good crest showing if they are going to get much of one. Some have vaulted skulls too which makes their crests look even bigger. In my Whites I get some that have wimpy crests, which is good for those ones to be able to see. LOL Dixie the Silkie Bogtown got from me is a Buff and she never had much of a crest and she could run all over the place. It is likely why she can stand up to the bigger birds if she needs to at BC's, she sees them coming