The base of the stem of a tall fescue plant is a purple color. If the leaves of the plants are purple then you could be deficient in phosphorus, especially if you have a sandy soil. Leaves also turn purple when they are wilting. Urine will usually leave a baseball size dead spot with dark green plants growing around the outside. Check your soil test if you've had one done for phosphorus deficiency. The greenish black stuff is probably alge, happens a lot when reseeding a lawn especially shaded areas. The alge won't be a problem once the grass gets up and starts to tiller out it won't be a problem.