Get a real blackboard made of slate and quality chalk. Much higher resolution than markers.
I'm glad they work for you, but out of curiosity, does anyone else find... just about everything about blackboards unappealing? The feel of chalk scratching on the board, the sound, drawing the characters (making curves feels awkward to me), even the chalk dust. I prefer whiteboards in almost every way except for markers drying out. It's one of those things I've kinda never understood why so many people prefer chalkboards.