The test bottoms out at 30ppm. So if you have less than that, it would look like zero.
BUT, the test is meant to be done in outdoor bright lighting, not indoor lamp light. You should redo the test during the day. High noon sun works best. Back to the sun, tube at waist height, and only two fingers pinching the very top of the tube (you do not want to obstruct the entry of light).
Redo the test before making any chemical additions.
It makes a big difference. You want your back to the sun with tube down at your waist like JoyfulNoise advised. Under a can light or facing the sun the black dot can stay visible longer and give a false low reading.
The two (sunlight versus a can-light) can not even be compared. A bright sunny day (in indirect sun) has a luminous output of nearly 11,000 lux (>1000 foot-candle) while standard indoor lighting has a luminous output of 150-250 lux. Even an overcast day has 4X the luminous output of a ceiling light (~1000 lux versus 250 lux).
The test should be done outdoors in bright lighting to achieve proper results. As for recommended levels, please see this Pool School Article - Pool School - Recommended Levels