Making sharp, clean lines on metal requires a sharp, clean point.
A scriber is made from hardened and tempered tool steel. To mark a line with a scriber, draw it towards you, and keep it angled in the direction it's going to travel so it doesn't dig into the surface being marked.
Dividers are used to mark up circles and arcs. They are held at an exact angle by an adjusting nut. Because of the sharp points on their legs, they also give an accurate way to transfer measurements from say, a steel rule, across to the work. Pin point the lines on the rule and make sure the circle has exactly the right radius. One way to make sure this leg of the dividers stays put on the surface is to make a small dent with the prick punch, before starting to draw.
On some hard or shiny surfaces, scribed marks can be hard to see, so marking-dye helps. A thin coat is applied to the area being marked. It dries very quickly. Then the path cut by the point of the scriber is easy to see.
Engineer's blue is similar to marking dye. It comes in tubes and it's a bit like blue butter. Sometimes in checking a fit, or testing for flatness, it's difficult to see the area being worked on. By smearing a small amount of engineer's blue on a surface plate it can be used to indicate if the surface to be tested is flat.The blue marks on the housing indicate the high areas which must be removed to obtain a flat surface. A flat surface would be blue over the whole surface.