A blue law is a law who's purpose is to enforce religious standards. Most commonly they are restrictions on what can be done on certain days of the week, such as preventing liquor stores or car dealerships from selling their products on Sunday. While many have been repealed or declared unconstitutional, some such laws still remain.
Within the United States, Blue Laws have been found to be constitutional in the decision of McGowan v. Maryland because regardless of religious origins, the harm done is not religious harm but generally economic harm. So long as such laws can be supported by even the flimsiest secular rational they are said to not violate the first amendment.