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.
Scratchy chalk is low-quality chalk or poor technique. Every writing instrument has its own techniques. Like how fountain pens work better at a lower angle and lower pressure, and ball-point pens work better at a higher pressure and the angle doesn’t matter so much.