If this is the case, simply move your plants to a more shaded area. If, however, black areas appear and grow larger, a bacterial or fungal disease could be the culprit. Remove any diseased areas, dry the plants and treat with a fungicide. As you likely know, good air circulation is important in orchid culture. When you do water, try to avoid splashing water from one plant to another.
Bacterial rot causes a blister to form on the leaf, followed by the rotting of the entire leaf.
In addition to removing affected parts and treating with a fungicide, try to lower the temperature somewhat and decrease the humidity. Given our hot, humid weather, this might be the real culprit. Black rot is a fungal disease encouraged by poor aeration, too low a temperature and excessive humidity. This causes dark, then black spots to appear at the base of the foliage. Other rots are prompted by a medium that's too wet.