Low blood oxygen levels, at what number would be low enough to cause a person to fall to the floor and have a blackout?

Depends. Its the rate of fall thats more important than the actual number. When the partial pressure of oxygen falls below 55 or 88% hemoglobin saturation the delivery of oxygen to our organs begins to decrease. If your organs, like the heart , is already damaged this drop can be very dangerous. If the organs are healthy they can tolerate this drop in oxygen levels for several minutes, sometimes even longe.
Depends. Depends on how long the oxygen levels have been low and the body has had time to adjust to it. Someone with a long-standing lung disease can walk into the office with a saturation in the 70-es, whereas someone with normal lungs will probably black out at that level. It is all individual.