[CliffordK (OP)]

I've been doing a project with collecting spring water in barrel.

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Even though I tend to have generally good water turnover in the barrel, I started noticing a significant algae growth in my barrel.  It doesn't really cause much harm, but it could clog my sprinklers and hoses. 

I could kill the algae with a  few drops of bleach, but I'd prefer staying away from chemicals.

I like the translucent barrel as it is fairly easy to see the water level, but it got me thinking, what if I chose a blue/black barrel?  This is actually somewhat important as I was planning on scaling up my storage system.

Anyway, it might also make a good science experiment, using translucent barrels vs opaque barrels for rain, or other water collection, and watching what grows inside.

[RD]

opaque will reduce algae ...

Rainwater ... Tanks are usually installed above ground, and are usually opaque to prevent the exposure of stored water to sunlight, to decrease algal bloom.

http://en.wikipedia.org/wiki/Rainwater_tank#Tanks

UV light does reduce the levels of bacteria ... 
http://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation#Water_sterilization
but visibly transparent/translucent plastic may block UV.