‘Blue’ is a term of impropriety. ‘Blue’ language is vulgar. Anything ‘blue’ was not to open on a Sunday. Drinking, gambling, and such. Even after supermarkets would (reluctantly) open on Sundays, Liquor/alcohol stores still remained closed. Repealing the Blue Laws means all the sinful stores are now freely open on Sunday.

“Blue Laws” were created for religious reasons specifically to promote the observance of the Christian day of worship.” -wiki