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: 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.

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.