im modelling a low poly human, and i have just started modelling the legs. the legs have just suddenly become blue. i am in the solid view. PLEASE help.
start taking some deep breaths 
probably, you went into weight paint mode–just select object mode from the menu on the 3d view header 
Or also if you are in wire frame view in Edit mode the polys are blue transparent and become purple where faces overlap in the view port.