What is it with vanilla dudes and butt sex? It’s like the one kink they allow themselves to have. Jealous too? Yikes. That’s always a sign of low self confidence.
A little piece of advice: if you do open this Tumblr side to him, I suggest you start a brand new blog when you do it, especially if your blog or comments or messages would give him reason to be mad.
I do think it could be very healthy for you two to share this experience together and it will probably help him discover some kinks, refine them, should he have kinks. It is odd to me that he doesn’t like porn; that normally is not a good sign regarding his libido, so just be prepared that he may not get into it at all and then you’ll be suspect to have your phone looked at all the time, just for masturbating.
I do think that he should still be open and understand that you need release and going to a porn blog is not that big a deal. Most guys would find it sexy as hell to have their woman on Tumblr rubbing themselves into a frenzy every night. I know I would.
I would roll the dice and show him the blog and Tumblr I’m general. Your sex life can only improve and if he gets mad you look at porn then he is just being a baby. 40 year old men should have more shit to worry about than being jealous over nothing, in my opinion.