Unless you are going to "Show" the dog, don't worry about it. As was already mentioned, in Dog Shows (bench shows) a small white star is allowed but not desired -- It does not have anything to do with "purity".
The white patch comes from one of the original ancestors of the Labrador, namely the St. John's Water Dog.
Here is a rare picture of the St John's. Back in the old days, dogs with the white star were considered to be the best hunters.