Well, going dark doesn't mean you can never see or speak to your H again. Obviously when you are married there are times and circumstances when you have to see or speak to each other. I suppose everyone's "going dark" would be a bit different.
In my case, I stopped calling him, period. I used to wait until I couldn't stand it another minute then call with some flimsy excuse or I would say I was just saying hello, how are you doing. I stopped doing that. After a couple of months of that he started calling me to say hello. I was dumbfounded the first time that happened!
Sometimes we did have to contact each other for financial reasons but whenever possible I let HIM do the contacting and then I was very impersonal. Not cold and rude, but just very nonchalant. Don't think THAT was easy! If I had to call him and I ONLY did it for financial reasons, I was still impersonal.
My C said to me one day "Have you noticed that since you stopped being so available that he is seeking you out and reaching out to you more and more?" Well I thought about that and it was true! One day many months later I said to him, "Why do you call? Why don't you just say goodbye?" and he was silent a minute and he said, "You know, I asked my C why I didn't do that and she said, Because you're afraid if you say goodbye, she really will disappear."" And I said, "would you even care?" and he said, "yes." Just that simple yes was enough. He has told me since we have been back together that he could not stand the thought of me disappearing. But he did not want to tell me at the time. He told me alot of things later that he said he was afraid to tell me at the time for one reason or another. Your H is probably the same.
You don't have to tell him you are going dark. Just ease off on calling him. If he calls and says he is coming round to work in the office say fine. Then go somewhere while he is there so he doesn't even get to see you. Now listen, PL, this stuff is NOT easy, as I'm sure you know!! I had an awful time doing this because I wanted to see/talk to him any chance I got. But I figured I had to make it worse before it could get better. But don't TELL him what you're going to do; just quietly and calmly do it. If he says anything, tell him the truth. Tell him it hurts too much to be with him knowing he is thinking about someone else and you need to take a break from it. There's nothing wrong with that and he can't argue with it. He's doing what he wants; you have the same right.
I hope I have made it a bit clearer. Do you have a C? You might greatly benefit from one. Mine saved my sanity because for a few months I was close to going off the deep end. Jealousy eats at you until you feel so sick and that crummy awful feeling you have NEVER goes away. I hate it! I hope the rest of the month goes okay for you. I'll be thinking about you!