Blood is actually a very strong coloring agent: it only takes a little to color a lot of tissue. In this way, it is a little bit like Kool-Aid: just a small amount of the powder can color a quart of water very easily! Remember the last time you made some Kool-Aid, and you were pouring the power into the water? It turned color as soon as you put any in! Blood can do the same thing. In fact, the hemoglobin in our blood (the molecule that makes it red) is called a "respiratory pigment" because it is such a strong coloring agent. (Incidentally, there are other "respiratory pigments" in the animal world, some of which are green. Just thought you might like to know!)
The fact that blood is such a strong coloring agent means that only a few drops of blood will create a very large bruise. The picture below is a patient who broke their radius (a forearm bone; see Anatomy Level 8 for an illustration) just a few days earlier. It was a very minor fracture, and she was in a small splint. I told her to keep her arm elevated, so the blood that came out of the fracture settled down toward her elbow. (If she had not kept it elevated, it would have hurt more and the blood would have settled down in her hand.) You can even see where the slight pressure of her bracelets kept the blood from settling in certain areas of her skin.
The ecchymosis may look scary, but it should not alarm you. It probably represents only a very small amount of blood, almost certainly less than 1/10th or 1/20th of an ounce! The body will process this blood and make it into new blood cells. The color will go from purplish-blue (above) to greenish-yellow to pale brownish-yellow to normal skin color, usually in a matter of a few weeks.