Has anyone experienced a black lab suddenly getting hailstorm type white spots ? What could be the cause of this ?

Can you be a bit more specific on where these white spots are? Do you mean their hair is turning white? or their skin? How old is your black lab?

Yes the hair has turned white in a hailstorm type pattern on both shoulders. It has done this over the past 6mos. It's a 3yr old black female, healty, happy, girl. Haven't seen anything like this happen on either side of her pedigree. It's like a splattering of white spots in a large area over the shoulders.

Google vitiligo and see if it applies. I think it can affect hair as well as skin, but not 100% sure.

I looked up vitiligo and it seems it affects mostly the face areas. While she does have a few splatter marks on her face most is covering the entire shoulder area with the same pattern of spatter equally on both shoulders.

This article says the animals will commonly have patches throughout the body:
http://www.professorshouse.com/Pets/Dogs/Health/Articles/Why-is-My-Dog-s-Nose-Turning-Pink/

A couple articles I read on vitilgo basically said that snow nose and dudley nose was the same thing as vitilgo, so I wouldn't trust those sources.

She may be a "Hailstorm" or "Snowstorm" labrador. It talks about them in "Reaching For The Stars" and has
a picture showing a Hailstorm labrador. Mary Roslin-Williams talks about these dogs in her book. These markings look like what is on this girl. Maybe the mystery is solved.

One of our bitches just had that similar thing happen. What is the name of the book?

Reaching for the Stars by Mary Roslin Williams.
It's called the Hailstone labrador.