Yes, and she 'pranks' you.

You hand her the marker. After all, it's only fair. You turn around and expose your back. You try to discern what she's writing, but it goes way too fast.

"Done," she states, "and now for your reward."

"But what about the writing?"

"Oh, we'll get to that soon enough."

What follows is essentially a carbon copy of what she did earlier. The same sensuality, the same eagerness, the same... wait. This is the part where you should experience glorious release, but instead, the pressure keeps on mounting. Your moans are a gradually becoming more like screams of pleasue, lust, and the good kind of pain. You wonder whether that's got something to do with the marker's magic. What is it going to take for you to get there?
It is at this point that you notice somthing in her eyes that wasn't there, earlier: curiosity. You know that she knows what you're thinking right now. She's looking or waiting for something. But that something doesn't seem to happen, because she suddenly pulls her head off your rock hard, throbbing member.

"Something the matter?" she asks, feigning innocence, "Here, let me help you." She removes her bra and holds it up to you. Your arousal doesn't help your confusion. "Go ahead, put it on."

Do what now? How is that supposed to help?

"If you trust me then do as I say", she asserts.

You wouldn't dream of anything like this - alright, you would. But still, you're able to put two and two together and decide to listen to her. As she sees you move to do so, she goes back to work. And with a passion. Somehow, she manages to lift you to even higher heights than before, and no sooner does the last strap rest on your shoulder than your cock erupts. And again, she makes sure to swallow and clean up every last drop of it.

Once your senses return to you, you don't really need to ask what she wrote, so you just ask "Really? What kind of a 'joke' is that?"

"It's not a joke. Well, it kinda is. But it's also not. I used the computer earlier and guess what greeted me when I opened the browser?"

That hit you like a truck. You were usually so careful to use incognito mode. This time you must have not. "I can expla.."

"No need. I'm not angry. Just a bit sad that you didn't share your fantasies with me. I love you. I want you to be happy. Do you really think you can't bring this into our relationship?"

"Well, I'm kind of ashamed of it. A man isn't supposed to like these things."

"Please know that you don't need to be ashamed of anything with me, alright? Now let me wipe that off. We don't want you to have to wear a bra to be able to cum, now do we."

"And I don't want you to be **** to like giving blowjobs", you say as you wipe her forearms clean, and trying to steer the conversation away from that particular topic, you add "Now what do we do with the marker?"

"There are literally endless possibilities... But let's tackle that question tomorrow. I'm really quite tired."

You don't object. But as you soon after lie in bed together, it takes you a while to fall asleep. Too intense was this evening, first with the magic of the marker, and then with your secret being out like that. Some time later, however, you do drift away.