Lavender hens bred to a pure black male can only give you 100% black in color offspring that carry one copy of the lavender gene hidden, these birds are what are referred to as \"splits\". If you are hatching lavender (gray chick down) chicks from this cross, your \"black\" bird MUST be carrying the lavender gene hidden, and is therefore a split.
Lavender hens bred to a \"split\" male, then, will give you the following offspring:
50% lavender chicks
50% splits.
If you are referring to a white wing primary on black chicks, that is typical for both pure black and \"split\" chicks and will grow in black once the adult plumage comes in, unless the melanizing genes are insufficient to cover that white completely.
Hope this helps.