BGA were among the first producers of Oxygen on Planet Earth. It's because of the rise of BGA that we are able to breathe Oxygen today. Later, plants figured out how to make food in the same way that BGA does, and then
they started producing Oxygen. Most algae are plants, and during the day they are net Oxygen producers, just like the higher plants.
BGA is a microorganism, a bacteria. The cells produce enzymes known as "cyanotoxins". These are various groups of enzymes which may be neurotoxins, which attack the nervous system, hepatoxins, which attack the liver, and cytotoxins, which attack individual cell walls. So the lethality of BGA has less to do with Oxygen, and more to do with enzymatic attack, although having said that, hypoxia would occur at night due to blockage at the surface and due to mass uptake if the algal mass was astronomically high.
This is the nature of the toxic condition in waterways known as "Red Tide" (named after the color of the marine algal blooms which are red), which results in fish kills, and which is dangerous to humans. There is little danger in our tanks however. You would need a few inches thick layer of BGA to leech enough toxins and to block enough Oxygen at night to be harmful.
The OP is reminded that there is a difference between algal blooms in the tank as opposed to simply having algae present in the tank. Blooms attack plants as a direct result of ill health, normally as a direct or indirect result of environmental conditions which lead to malnutrition. Therefore, when there is evidence of an attack, steps should be taken to alleviate the malnutrition, otherwise the plants will continue to fail. Small populations of algal colonies are often unavoidable, however these small colonies, especially if they are on hardscape only, do not pose a danger.
Cheers,
