Because it betrays a feeling of shame, which is deeply personal and private and when others see it it can appear as a weakness. That being said, many people don't think it's a big deal but I think most people become embarrassed about blushing because they feel emotionally vulnerable (again, weakness).
I think the other party is not going to really care unless you over think it or bring it up.
Please tell me why? It is an natural emotional response and to me it is normal. I see it and do not really think anything at all.
Some people (like myself) find blushing to be an unwanted response, it also has to do with the extra attention you receive when you blush. I always hate it when I blush because it sometimes makes the person I'm talking to point it out and make a big deal about it, I find it kind of embarrassing.