What causes a white spot on chest of a black lab puppy?  The mother
was yellow lab and father was chocolate lab.

We have a four-year-old purebred Black Lab. When she was a pup, she
had exactly three white hairs on her chest, arranged in a perfect
equilateral triangle! We were disappointed that the white hairs
disappeared as she matured.

The only drawback to a white spot is that the American Kennel Club
considers the presence of any white hairs to be "permissible, but not
desirable," so a Lab with a spot is at a disadvantage in dog shows.
Our veterinarian has told us that approximately 25% of the Black Lab
puppies she sees have a few white hairs. The vet told us that this is
due to the genetics of the pup's ancestors, and even though neither
parent may have a white spot, a combination of genetic factors make it
possible that their offspring may have some white hairs.

Here is a good technical explanation from a site which has a lot of
interesting information about coat color in Labrador Retrievers:

WHITE SPOTS

To analyze the reason why some black Labs have only a few,
not-easily-seen, white hairs on their chests while others have small
white spots, it is best to first picture that all Labs are white--the
condition of having no melanin production. The gene loci for color
control both the color of the pigment as well as the distribution of
melanocytes throughout the body of the Lab. Therefore, in a black Lab,
although color is determined by alleles at the B locus, alleles at the
A and E loci determine even distribution of the color over the entire
surface of the coat. Labs that carry an allele other than "As" at the
A locus, have a greater likelihood of expressing more white hairs than
those Labs that do carry "As". Therefore, although all Labs should be
homozygous for the S allele at the S gene locus, some may still
express white hairs on the chest, bottom of the feet and under the
arms and groin areas.

Labbies.com: A Detailed Examination of Coat Color Genetics in the
Labrador Retriever
http://www.labbies.com/genetics2.htm

These sites offer further information about the genetics of coat color
in Labs:

Cedaridge Kennels: Lab Colours
http://www.geocities.com/Heartland/Acres/4603/labcolours.htm

Ashstone Labradors: Coat Colour Inheritance in The Labrador Retriever
http://members.tripod.com/~Ashstone/colour.html

My Google search strategy:

"labrador" + "retrievers" + "white spot" 
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=labrador+retrievers+%22white+spot

Thank you for an interesting question. If anything above is unclear,
or if any of the links do not function, please do not hesitate to ask
for clarification.

Best wishes,
pinkfreud

The question was answered very well and gave additional web sites to
inform me more about the subject.  Thanks