Years ago, I helped my younger sister make a black tablecloth that we used for her science fair project. Now it is very useful for chalk art! It is a rectangle of black cloth with two squares sewn on one side with a gap in the middle. It was designed to create dark but visible space under a table. I can hang the tablecloth over my easel and lighthood and create a "tent" to draw blacklight under in a bright room! I read about your family having to use flashlights while you drew your background and blacklights, and this might be a solution. It also helps when you can't cover the windows in a church or auditorium - you can cut down on the glare and create a curtain effect.
Clear as mud?