Help installing ceiling fan in old house
There are almost certainly more colored (non-white) wires in the back of the box which are for other hots.  

Is the blue wire blue or green?   It matters a lot.   

A green wire should be attached to the ground wire bundle that may be in the back of the box.  Absent that, look and see if the wires enter the box via cable or pipe.  If cable and no ground wires, connecting a ground is pointless.  If pipe, most likely this conduit is the ground, and the green wire  can be attached to the steel box.  There may be a tapped hole for a 10-32 screw somewhere in the back of the box.   If not and you are crafty you can drill and tap one (any machine screw 6 or larger will do as long as the thread is at least -32). 

If the wire is honest-to-gosh blue, then is it light blue?  That might be a Euro neutral, but that would be strange.  Otherwise it is assuredly one of the hots, you have a separate hot for fan and light, allowing you to control the two of them individually.   If you want, you can do some extra wiring to exploit it... Or you can simply connect the blue and black together to the power supply,  and fan and light will operate together.    Some fans permit local control of light and fan i.e. With 2 pull cords.  Those are always fun!