I would say that if you're barebacking during menstruation and the blood is flowing freely, then yes, that might be a higher risk that barebacking without menstrual blood present.  HIV is present in menstrual blood if a lady is infected with it, but it is also similarly present in regular vaginal lubrication fluids.  I think that the factor that MIGHT make menstrual blood a higher risk factor is the idea that menstrual blood may have *higher* concentrations of HIV compared to regular vaginal fluids.

However, there are lots of ways to have sex during menstrual flow that do not involve increased exposure.  Here are some ideas:

- have sex with a condom (a female condom is even better since instead of stroking in and out of her actual pussy you are stroking in and out of the relatively stationary condom... if you're using it right) so the blood doesn't contact your cock.

- use sexy black latex or nitrile gloves and water-based lubricant for digital penetration if you have any small cuts or burns on your hands (you can use lemon juice or any other mildly acidic liquid to test for them if you want to.)

- the lady can use a sea sponge or beppy sponge to stem the flow by wetting the sponge, lubricating it, pushing it up to her cervix and then adding extra lube to make up for the fact that there is a sponge which may absorb some of her natural lubricating juices.  the sponge will absorb most or all of the bloodflow so that it is not a factor during your time together.  this method is flawless when paired with a female condom, unless the lady is flowing very heavily.  beppys are more absorbant than sea sponges, but sea sponges feel more natural inside the lady and on the head of the cock.

- avoid oral contact with menstrual blood, particularly if you have any cuts or sores in your mouth, have a stomach ulcer, sore throat, etc.  (but much like with vaginal fluids and semen, as long as there is no entry to your bloodstream in your mouth or throat, then HIV is denatured/destroyed pretty much instantly upon contact with your stomach acid, so swallowing is actually not unsafe in and of itself.  your freshly-brushed gums are your riskiest point of entry for HIV in the mouth, so skip the toothbrush and use mouthwash beforehand instead.)

I don't think it's a significantly riskier experience if you're already taking basic precautions to protect yourself from HIV.  Please remember that HIV does live in vaginal lubrication and if you are penetrating partners with your unprotected fingers which might have tiny cuts in them, then you are already exposing yourself to the risk of HIV transmission through cuts in your fingers, regardless of the presence of menstrual blood.  That's why I always use gloves when I digitally penetrate someone else, especially anally.  You never know someone else's status, so I always take whatever steps I can to play safer and protect myself (and my other partners)!

The more you know!!