I know veins carry deoxygenated blood, wich is why they are blue, but why would they seem darker/more noticible some days as opposed to other days?

Many factors.: Veins constantly change in size depending on many factors including level of hydration or dehydration, pulse rate, cardiac output, hot and cold environment and even skin color and texture. Since blood is always in a dynamic(moving) state, all of the above factors can affect the size of the veins as well as the apparent color of the venous blood.