Test #2 (more accurate): set your buoyancy to neutral at 10m
99.9% of blackouts happen on, or within 10m of, the surface.
Therefore, if you set your buoyancy to neutral at 10m, you are almost certain to float to the surface in the event of a blackout.
Being positively buoyant will also make it easier to ascend the last 10 meters of a difficult dive.
To set your buoyancy: dive down to 10m, gently let go of the line, and check that you neither sink nor float. If you sink, return to the surface and remove some weight.