A water change might get a few additional cells out that have left the substrate and are basically plankton at that point. It certainly dilutes the number of cells, but the overall success of the treatment appears to be dependent on many factors, a lot of which seem to be unknown.
Some say dinoflagellates just consume nutrients and make it look like they are bottomed out, others say bottomed out nutrients cause dinoflagellates. Others have dinoflagellates during elevated nutrient levels and depending on the species getting them into the open water column can be difficult, even with a blackout. I'm always searching for dinoflagellates and even if your substrate and the rocks appear to be free of them, some cells might just sit in some apparently dormant state on your backwall (
see second image on page 1, post #3) until conditions become more favorable again.
I wish you good luck with your blackout attempt, but don't get discouraged if it doesn't work out, you can follow up with other treatment methods like silicate dosing or nutrient adjustments (in either direction up or down, I prefer to start with down first and if that doesn't work
ammonia dosing, and
phosphate dosing is a cheap way to add nutrients without increasing organics which might play a role as well).
I think 3 days is the commonly recommended duration, but some people say it actually takes longer and recommend 5. I have never really done any blackouts so I'm not sure how well corals are dealing with absolute darkness for that long. I'm sure you will find articles and reports here on R2R about blackouts.
