Blood isn't hard to rub off and you can rub the color off a a Marans egg. It's something mentioned all the time. When a bird lays with speckles they can be removed because it's the color it was painted on the way out. I can do it with my Marans and with my Wellies, spots.
Obviously no one that has told you this is blood, owns a Marans. If they do they do not know the details of the breed or anything about they're egg color process. Prick your finger and wipe it across a white egg. That's what blood looks like. It smears, because it is rubbing as it's pushed out. You can absolutely tell if an egg hhas blood on it. Marans included.
You're in the best possible place to learn about your breeds, right here on this site. There's a learning section, breed section and thread dedicated to most of the popular or rare breeds. Join in or just read and you'll find all of your answers.
It's so hard to tell someone what's going on, when there are so many that think they know and jump it. Seems to be happening a lot lately. Maybe I'm just now starting to post on those, because I've noticed even before I ever signed up to this site, that people will take the wrong answer as truth, and not even give the people who are experienced a chance to give a correct answer, Then you have one more person who decides to pass the wrong information off yet again to even more people, so instead of helping they have others believing all the wrong information.
Don't believe me? Look up Americana on this site and see how many people don't even know what breed they have,because someone sold them something they thought they had.