He looks like a robot. Robots are electronic. Electricity can be blue. Zekrom has some blue in his design. It doesn't necessarily have to be yellow to be electric. Have you ever seen lightning? Lightning has tints of blue in it.
Reshiram has a possibility of being part fire seeing as there is a torch for its tail. Maybe it controls volcanoes or actual fire.
The reasons why the legendaries for Black and White could be fire and electric:
Fire--you light a candle to see in the dark. It's light in darkness.
Electricity--you turn on a lamp when it's dark. It's also light in darkness.
Therefore, Fire and Electricity tie into the light/dark aspect. Both create light when there is darkness. It doesn't have to be so literal and be part light and part dark types. The legendaries could be part fire and part electric and still convey the same message. The titles make sense, because Black represents dark and white represents light.
Also, with fire and electricity, not only can they bring light, they can cause darkness as well. If you use too much electricity, it causes power outages (like the one in Sunyshore). Without power, you're in darkness. Fire also creates smoke. You can't see when there is smoke and ash. Therefore, there is darkness.
Fire and electricity control light and dark. Therefore everything I've said makes sense for the legendaries to be part fire and electric.