In my opinion I don't think your going to be able to get the pond dark enough to kill or keep plants from growing. You might be able to slow the growth by shading, but killing or keeping plants from growing is not possible. Since you mentioned coontail, you know this plant isn't rooted so it floats near the surface or can move into shallow water where it will be able to get light. This website states coontail can grow in low light conditions as well: http://www.mciap.org/herbarium/Coontail.php
If you can see through it, then light is definitely making it through the water column. Plants really don't need a lot of light to grow and once they make it to the light attenuation level they will spread laterally. This is why hydrilla has 75% of its biomass in the top 1/2 to 1/3 of the water column.