Blessed chalk?

Larry and I often talk about Bohemia/Germany/Luxembourg/Poland in the context of Catholicism, and how my ancestors were affected by it.  Did they fervently believe, or was it really just another obligation?  Since we were both raised Catholic in the era of almighty nuns and priests, theres a curiousity about which customs and holy days were brought to America by those Bohemian German Catholic Heschs and Polish Catholic Otrembas...and which customs/holy days were just 'central Minnesota' or Benedictine.

This afternoon, we chatted about amulets and superstition, but I didn't remember anything in particular.  Larry asked about stuff over the doorway--did we do that?  I said blessed palms and blessed chalk.  Umm....Chalk?
WOW.  Even I hadn't remembered chalk until he asked.  Where did that come from?

Once more, Google to the rescue: a central European custom at Epiphany--using the three kings' names with the year...so this year, you'd write over the door in blessed chalk:
20 + C + M + B +10
But, WHAT made them connect the Magi with chalk and doorways?  Sigh!

(LOL...I do remember it as being able to write on the walls, and that mom couldn't object!)