Let's say I want to buy a specific loaf of bread called "Blocky". How should I ask the question about whether the store has it "in stock".
I tried "Do you have a Blocky loaf", but I am not sure if I am right.
Do you have any Blocky bread?
asks if the seller has any of that bread at the moment.
Do you have Block bread?
usually means Do you have any at the moment, but can also mean:
Do you carry Blocky bread?
which means: is Blocky bread something that you sell (though it may be sold out at the moment).
As to your suggestion :
Do you have a Blocky loaf?
specifically asks if the seller has one Blocky loaf
Customer: Do you have a Blocky loaf?
Seller: We have several, which one would you like?
That phrasing is also commonly used in :
Seller: Do you have a Visa card?
Customer: I have two.
Seller: Either one will do.