When i was in high school and wasn't having dental cleanings every 6 months as you're supposed to, I was eating an apple and freaked out when I saw a bit of blood on it but I figured out it was my gums that had bled. If you don't get regular cleanings, that blood could be from your own gums. Just brushing with a toothbrush daily and flossing isn't good enough to protect your gums from gingivitis and sensitivity. You must get cleanings every 6 months to maintain good dental hygeine. And while pregnant, our blood volume increases and we are more susceptible to bleeding with the slightest of pressure. Press on your gums and if they are red or bleeding, I'd recommend a dental visit. But nothing to worry about as far as hiv is concerned. That is a rumor :)
This is unlikely to be injected blood into the banana. And the strange instance that it may have been, it is impossible to infect someone with HIV in this manner. This should not be something anyone worries about. good luck