Oh crafter, you did not offend me at all.
We all have different experiences. I know inks can run, even Sharpies.
It is just my experience that if they are set with heat it helps, but a water spill will still make the red & green run.
I don't use steam when I press blocks.
The point is...a straight line, made with whatever mark, keeps the block from being wonky.
Happy sewing

crafter, I just reread your post. No, I wouldn't use a Sharpie on a quilt either.

There is another Sharpie product that is specifically a "Laundry Marking Pen". It is a bit more expensive than the standard Sharpie, but worth it for being sure it will stay and not run.

Great Idea!!
Something new everyday here!