Ms. Blue hasn't posted in awhile. I'm hoping she'll check back soon.
For starters, "cheating" isn't ANY good advice and gladly she's smarter than a couple posters here. (She's also smarter than the horndogs that want her to be close to them to "help" her. She's looking for help here. Give her a break!)
It is concerning that Mr. Blue has lost his libido. BTW, Viagra, Cialis DON'T address lack of libido. There has to be some desire for those chemicals to work. So disregard those comments. Ms. Blue should have a discussion with Mr. Blue about getting a full spectrum checkup to find out if there isn't some physical reason for his lack of interest. (And the poster that said he didn't want to have sex with her isn't what she wrote. He might not be interested in sex with ANYBODY. If that truly is the situation, it might be physical or psychological and should be properly address outside the amateur aspects of this forum.)
Once the potential medical issue is addressed, then next is to open up communications regarding your fantasies and "needs". Before you can turn your desired fantasy into a reality, you'll need to discuss them to find out what his ideas are on the fetish. If he is concerned with the possibility of "losing you" to someone else, then that will be a difficult discussion. How can you convince him that you'll be "playing" with others, but "staying" with him? You'll need to think about how to address that. Regarding the physicality of what you desire, it might be helpful to find a couple videos online that visually show those to Mr. Blue and find out what his thoughts are on them. Since "cuckolding" is a major search topic nowadays, he might be one of those who is so far secretly on board with your desires. (If so, great for you!) If there is resistance, you'll need to determine how to address again making him comfortable that you'll be able to handle his issues. Don't forget that if you find enjoyment in the activity, you want him to enjoy it WITH you. (That you've already mentioned is your intent.)
But stop one on this tour is finding out why his libido has tanked. Is there stress he is under (ie.-work, other family members, children, finances, etc.) that might be part of the reason for that? It has been said so many times that communications is so important with sex. The time is now to have some discussions (while taking time to masturbate in the shower during that process).
Good luck.