How are fly-by-wire airliners controlled in case of complete electrical failure?
For the complete electrical failure in the aircraft, the following systems have to fail:


The onboard power generation system, usually a synchronous generator has to fail in all the engines.
The APU (Auxiliary Power Unit) has to fail.
The RAT (Ram Air Turbine) should fail to deploy.
The batteries and static inverters should fail (this won't help in controlling the aircraft, just added for sake of completeness).


The probability of this happening is very very low, and can be discounted. It is more probable that some other system (for e.g, hydraulics) fail before this happening. So far, to the best of my knowledge, no complete power loss has been reported in any fly by wire aircraft.

The reason the Airbus aircrafts are controllable in case of electrical failure is due to the fact that the fly-by-wire system has multiple redundancies that offer layered protection:


Normal Law offers all the protections (like stall, load factor etc). This is the system in operation during normal flights.
Alternate Law offers limited protection including stability, load factor and stall protections
Abnormal Alternate Law limits the load factors and offers yaw damping only
Direct Law is the lowest level of fly-by-wire system and directly transmits the pilot inputs to control surfaces, while still giving stall and load factor warnings
In case all these fail, the system reverts to mechanical backup, where pitch control is achieved through the horizontal stabilizer and lateral control is accomplished using the rudder pedals. 


In case of mechanical backup, the system will operate as long as hydraulic power is available. In case of electrical failure, as long as hydraulics are OK, the pilot should be able to control the aircraft. 

However, usually, if there is no power, there is no hydraulic pressure (except reservoir, maybe) and the control is lost. In case of the Baghdad DHL aircraft, only hydraulics was lost and power was available.

There are some more points to consider in case of total electrical failure.


In case the control surfaces are moved by electric power (like 787), power loss means loss of control.
Power loss implies that the engine thrust cannot be controlled (of course, the engines won't be operational in that case anyway).
Some fly-by-wire aircrafts are unstable by design. In these cases, any power loss will lead to loss of control and crash.
I'm not sure there is any direct mechanical linkage in civil airliners. The amount of force required to operate such a system would be enormous.


Bottom-line is, in case of complete power failure, the aircraft is as good as dead. One example is a Fairchild Swearingen Metroliner (which was not fly-by-wire), that lost complete electrical power due to a lightning strike. The aircraft entered into a uncontrolled flight, which loaded the aircraft beyond limit, resulting in its disintegration.