"Blue-Green Algae" is a complicated topic. I'll try to make a few comments about it but there'll be more out there than I can cover for now. Technically, blue-green is not actually an algae but a different microorganism. You can read more about what it is and how to specifically deal with it in the planted section or via links they provide. (You've probably already done that to decide on the black-out!) Despite it not being technically an algae, its talked about in most of the Algae helper guides and like other "algae" it needs to be dealt with on its own terms, not general terms. Its causes will also be somewhat specific I believe.
CO2 is of course the main provider of the element Carbon (C) to plants, both terrestrial and submerged. Terrestrial plants can obtain CO2 orders of magnitude more easily than submerged plants. This is one reason why so many plants actually grow "emerged" (having leaves at the surface our out in the air) rather than "submerged." Carbon is the backbone of the sugar chains that carry energy around the plant and thus it is an absolutely crucial element. But getting carbon to plants so they can use it is anything but easy!
At normal atmospheric pressures, a certain amount of CO2 will dissolve in water at a given temperature. This is separate from how much O2 will dissolve, incidently. For a given level of water surface area and surface movement, there will be a certain amount of CO2 available. If the surface is more disturbed there may be somewhat less CO2 dissolved in the water. When the pressure is greater on the water, such as when it is in a city pipe system, the water can hold more CO2 than it can at atmospheric (out in the open) pressure.
There are currently 3 broadly practical ways that Carbon (C) is delivered to plants in a modern planted aquarium: A "pressurized" CO2 tank system is installed with a special "regulator" valve that bleeds a little CO2 into a device like a "reactor" to turn the CO2 into tiny bubbles to increase the percentage of it that gets absorbed into the water before it simply fizzes off into the air. This we generally call a "pressurized" system in the hobby lingo.
The second method is any of a whole bunch of variations of putting yeast and sugar in a container (like an old soda bottle) and then piping it into the aquarium and trying to also make the bubbles smaller. There are all sorts of commercial variations on this that make it more expensive but possible a little more convenient, but not necessarily. This are all collectively called "DIY CO2" or "DIY Carbon" in the hobby lingo. (DIY for Do It Yourself.) (These "fermentation" methods are the same methods used in home brew beer etc.)
The third method is called "Liquid Carbon" in the hobby lingo, and technically doesn't involve CO2 at all. Instead its simply a very complicated molecule (I believe its the similar or the same as one of the old photo fixer chemicals for film photography) that a few companies have figured out how to make and you buy bottles of it from them and dose your tank. The complicated molecule is something that plants (most plants but not all) can absorb and use at a later or different stage of their internal carbon cycle instead of the carbon they would have been processing had there been more CO2 dissolved in the water.
Pressurized CO2 is the "gold standard" for delivering carbon to underwater plants and is basically untouchable as the top method. Its got a quite high startup cost and is quite technical and carries a small risk in the home, but is pretty cheap in the long run, compared to other methods. DIY Carbon has low startup cost, is very hobbyist oriented and even fun initially but quickly drives you crazy with maintenance and other problems, so most people eventually abandon it unless they are very disciplined. Liquid carbon is expensive in the long run but totally easy in that you simply pour in a partial capful daily or on some schedule. But technically it can be challanging sometimes in that it can "melt" plants and even harm fish if not used with reasonable caution and understanding. And in some instances it simply doesn't work nearly as well as real CO2.
~~waterdrop~~
