Friesians (excepting the rare chestnut) are black, a commonly known fact. But not all black coats are created equal. Friesians actually come in a variety of shades of black, ranging from true blue-black to essentially a very dark black brown or bay. In the winter, this is likely unnoticeable, but in the summer, when the long hair is shed and the sun comes out, your previously dark black Friesian may end up turning a startlingly lighter shade.
Depending upon the individual horse, this may be due to just the basic coat color, or it may be due to prolonged exposure to the sun, or both. Normally sun bleaching is caused by a combination of an inherently brown-tinged coat with salty sweat and the summer sun.
On some horses sun-bleaching affects mostly the mane, forelock and tail, turning the tips of the hairs a sometimes startlingly bright red-orange or brown. In other cases, the entire body area of the horse (not the legs, or ears) begins to lighten to an almost dark gold-brown color.
Preventative measures can be taken to keep the coat color dark during the summer, the most effective of which may be to keep the horses inside during the daytime and put them out at night. This also allows the dark-colored Friesians to stay inside in the shade during the heat of the day which may help to keep them from overheating in the high temperatures of summer. Some people put sheets on their Friesians, though this does not protect the head or neck from the sun.
There also exist some feed supplements such as Black-as-Knight which claim to darken the coat, though results vary.
That said, there is nothing physically wrong with a light-coated Friesian, and they are not penalized at the Inspections for it, so many people are rightfully quite content to leave their horses their natural shade of (light) black.