Just picked up a quart of JDBB yesterday and my local dealer said he didn't stock the hardener because of too many complaints from customers. According to him the customers who have used it say it makes the paint dry way too fast. What effect that has I don't know.
Instructions on the can say to mix it 10 to 1 (paint to thinner) before spraying and the dealer said to mix the thinner in slowly as putting it all in at once can cause fish-eyes.
Are you sure the can you used said Blitz Black? JD does have a gloss black in the same kind of can. Also remember Blitz Black is not a flat color its a semi gloss so it will have some sheen to it.