Both big toenails discoloured, blue looking; right one particularly looks as though black streak in it. slight pain but nothing major. what to do?

Microtrauma: It sounds like your toenails are hitting the end of your shoe. This usually happens if the toe box is either too small (toe is pressed up against the end) or too large (foot is sliding around and the toe is hitting the end). This is very common in runners--it can lead to either thickening of the nail, or it may eventually turn black and fall off--a new nail will grow in within a year.