There are many things that blood can represent such as imminent danger or such as a strict time limit that must be adhered to. Blood also represents life. Dry blood takes on an entirely new meaning though, especially in regards to dream interpretation.
A dream about dry blood means you are reconsidering an old wound. These wounds are usually emotional in nature and this is why they are on your mind. You were hurt at some point and the realization of that as well as the fresh emotions that this realization made you feel are what contributed to the dream about dry blood. Even though it happened some time ago and the wound is healing or has mostly healed, there is still dry blood there as a remnant of what once happened.
Dry blood can also represent time having run out. Dripping blood means that you are on a time limit to solving a life or death situation and it could prove difficult if you aren’t able to achieve your goals soon. However once that blood has dried it generally means that your time is up, and probably long past. The blood has dried and you were unable to achieve your goal in time. Now the project must be abandoned and you must search for something new to accomplish.