I do not agree with removing the leaves with Black brush algae here. The Echinodorus (sword) plant is very adept at mobile nutrients, which means the plant can transfer nutrients from the older leaves (on the outside, and not surprisingly the ones usually with BBA) to newer growth (which is from the centre of the crown). Once the base of the leaf stem is brown, nutrients cannot move so then it is OK to remove the then-dead leaf.
Rectify the light/nutrient balance first. This algae is what I term "problem algae," unlike common green algae that is often not even seen but is naturally there. In a planted tank, all problem algae is cause by an imbalance of light and nutrients. Nothing else. So establishing or re-establishing the balance is how you deal with it. And the aim is not to eliminate it, but to control it. Which means, not on plant leaves which will eventually kill the leaf and the plant. I had it on wood and found it incredibly attractive and natural, as Uberhoust said.
The light involves intensity, spectrum and duration. The nutrients include fish being fed, water changes and plant fertilizers/additives. If you provide this data, we may have suggestions. I had this algae--which by the way is actually a red algae, which can be dark green, grey, black, or deep red depending upon species and circumstances--in some tanks. Once I got the balance worked out, it no longer increased and the tanks were free of it for the following 6 or 7 years.