I believe I understand your concern because similar confusion arose at some point in my English study, too, due to a linguistic difference between my native tongue and English. Your language is perhaps like mine, Thai, where you answer to the truth condition of the statement of the question rather than the truth itself. For example, if you ask, in Thai, "aren't you going to school today?" and the answer is "yes," the answerer means they are not going to school. "Yes" here is "yes, (it is true that I am not going to school)," not "yes, (I'm going to school.)"
I think it helps if you think of English as somewhat simpler than that. In English, whether you answer "yes" or "no" only depends on the reality and has nothing to do with how the question is worded. Whether the question is "do you know?" or "don't you know?," "yes" and "no" still have the same meanings: in both cases, if you answer "yes," it means "you know," and if you answer "no," it means "you don't know."
Answering with plain "yes" or "no," however, may sound too flat and could sometimes be misunderstood, so it's safer to respond with a complete answer like "yes, I do." (meaning, you know) or "no, I don't" (meaning, you don't know) if you can't think of anything fancier like "no, I never knew," or "no, this is the first time I've heard about it!"
As stated by other Answerers, one thing to consider is that sometimes "don't you know?" is a rhetorical question, meaning the speaker doesn't actually expect an answer, so you don't have to answer them. You usually can guess from context whether it is an actual question or not.
Asking :
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.
Best Answer
GREAT QUESTION
If you are asked
"Yes" means that you did go, and "No" means that you did not go.
If you are asked,
the negative form of the question almost invariably implies doubt or criticism or both. Answers of a bare "yes" or bare "no" do not address that implication. The answer would normally emphasize the mere substance of the reponse with
or
If no clarification is added, the usage of "yes" and "no" is the same as though the question were asked in the positive. To avoid potential social misunderstanding, however, ask questions in the positive and answer negative questions with a clarifying "I did" or "I didn't."
ADDITIONAL EDIT: In a comment below, reference is made to a response to a similar question. In that response, the point is made that rhetorical questions are frequently made in negative form.
is not attempting to elcit information, but to elicit agreement and confirmation. Anything other than an emphatic answer is likely to be ill received.
To summarize, questions in negative form usually have a social dimension that is usually addressed by answering with extra emphasis.