How does mg2+ block the effect of extracellular calcium on neurotransmitter release?

Magnesium is a strong anode than calcium. So, Mg will be oxidized into Mg2+. So instead of having Ca being the reducing agent, Mg will takes its place. That's how the "Blocking" occur.
Without Ca2+, no neurotransmitter was released because the exocytosis of the synaptic vesicles is dependent upon Ca2+.
So, when magnesium is added to the extracellular fluid it blocks the calcium channels and inhibits the release of neurotransmitter.

The angle of reflection is 30°

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.