Black beard algae is actually a red algae and thrives in high-phosphate environments. What this means is that if you have it in your aquarium, it's either a sign that more water changes are needed - perhaps boosted by using a gravel cleaning syphon, or that your tap water contains high levels of phosphate. As ever, a water test can save a great deal of guesswork and help you tailor a maintenance schedule that suits your aquarium.
Areas where old Victorian plumbing is still in use tend to be the worst for phosphate contamination, as water authorities add this to the water to prevent any corrosion of potentially harmful lead piping. To put this into perspective, black algae may be a nuisance but is preferable to a dose of lead poisoning! If your tap water tests positive for phosphate, it may be better to switch to RO water to avoid adding more algae-food each time you carry out a water change.
With inputs of phosphate minimised and solid waste removed from your aquarium, adding a phosphate removing resin to your filter is a good way to starve out the algae. Old fluorescent tubes can also be a problem and with time, their output can shift to favour the growth of algaes rather than more demanding aquarium plants. Unlike green algaes, few aquatic creatures have evolved to eat black algae but the Siamese Flying Fox Crossocheilus siamensis (also known as the SAE or Siamese Algae Eater) can be used as a means of control. The key to beating this, or any other nuisance algae, is to change the conditions in the aquarium with cause it to thrive.