Is it okay to make this kind of blunder during an interview?

That's fine, if you did well with everything else you should be able to skate on that one question, and you were honest so that's a plus, instead of trying to answer it with some bull you made up. Most people make at least one tiny mistake in an interview, it is a stressful event. I was being interviewed for a job several years ago and was having a hot flash, started pouring sweat off my face ( really awful !!) and still got the job. I wouldn't have even hired me on that day !! and yet they still did, so don't worry too much ~ and good luck to you.