There are DNA tests but they are extremely inaccurate! Don't waste your money. Many of them don't even properly recognize all of the many different bull breed dogs.
I have a friend who did a "good" one just for fun on her known purebred show dog and it came back as 4 different breeds lol
Many blue dogs have a tan hue to their coats. Blue is a dilute of black or seal. Seal is another color that looks like black but in the light they look almost chocolate, if your dog wasnt not a dilute she would most likely be seal