Making a dark, suspended chocolate sauce
1) Well, it happened because you used something that is rich in saturated fat. 

Saturated fat liquefies when heated and solidifies when cool (this is why some places call the type of confectioner's chocolate that doesn't need to go to the fridge to solidify "hydrogenated chocolate", because it is rich in artificially saturated - hydrogenated - fat)

2) My recommendation for you would be adding milk or if you don't want to lose the "darkness" of your sauce, you could add in vegetable oil and food emulsifier (or if you have it on hand, pasteurized egg yolks)

3) You could also change the type of fat you're using, hence the vegetable oil (unsaturated fat) suggestion