Many women find this disgusting, but it's actually a huge turn-on to be queened at all moments, including during menstruation or without being freshly clean. It's that time of the month when a woman is particularly sensitive and it can be extremely gratifying to receive oral pleasure despite the mess.
This provides humiliation, but is more of a certification of who's in charge and asserting female authority. Natural odors and even menstruation can be extremely erotic to him as a submissive, and it also firmly sets the point that there is no time he can't be expected to provide oral pleasure.
You can tease him by telling him he needs to clean it up, or simply let him know how pleasurable it is to you.
Make him give you pleasure when he absolutely least expects it…right in the middle of your period, after you’ve just gone to the bathroom, or after you've just exercised and you are still sweaty. The point is you should educate him to be ready to service you whenever you want, no exceptions.
It may seem disgusting to you, but it's actually not disgusting at all, and some men will find it extremely erotic. You are not the person doing it, so don't get disgusted…just let him go for it, and if you can, encourage him and make him finish you.