If it was caught early enough, the plant should recover. Yellow leaves on the inner parts of the plant indicate over-watering. If the newest growth is yellow, it may be because the water is too alkaline. Hydrangeas have a very difficult time with hot, dry air, resulting in brown leaves. Increase the humidity and move the plant to a cooler location.
Colors change in response to a change in the pH of the soil.
In alkaline soil, hydrangea blooms tend to be pink. In acid soils, they tend toward blue. Distinctive brown spots are probably caused by Botrytis. That disease is encouraged by allowing water to stay on the leaves for any length of time. Blackened areas can be the result of cold or damage from leafshine products.