Blue/gray is not a correct colour for an American Bulldog so if you want to breed or show, do not buy blue, if you just want a cool looking blue dog, go for it, here is a link to Cruz’ pedigree. 

Here is a link to the National Kennel Club Breed Standard.  LINK  It describes solid blue american bulldogs as disqualified. 

The American Bulldog Registry & Archives also disqualifies blue.  Here is a link to the ABRA Breed Standard https://www.abra1st.com/breed-standard/

The UKC also disqualifies blue, here is a link. https://www.ukcdogs.com/american-bulldog