A standard USB 1.0 and 2.0 cable has the following wires:
- data: D+ (green) and D- (white)
- power: +5V (red) and ground (black)
- shield - the non insulated wire
Image by John Beale.
Ground = subsystem ground, the point which voltage of the connected circuits is considered 0 (e.g. the USB hub ground, the motherboard ground).
Shield = device ground, makes sure there is no potential difference between the two connected devices (e.g. the PC metal case). It ensures screening. Sometimes it is not connected to anything else but the USB connectors. Sometimes is the same as USB ground.
I guess that the blue wire is not used for data, it is the equivalent of the black wire (GND), and the black wire is the shield. Or the blue wire could be the shield.
You can use a continuity tester and check each wire against the USB connector - when you have a connector. When the wires are connected directly to the PCB, you may get an idea about what they are for if you look at the PCB. No one can be sure what the wires do when the manufacturer doesn't follow the standards.
Personally I came across this situation (USB mouse):
- green and white were the data wires (standard)
- blue was USB ground
- orange was +5V
- no shield