Left arm vein is blue and in pain, what should i do?

If you feel: Also that the vein is firm, can't be collapsed you may have a clot in the vein. Did you give blood, was blood taken for a test, was the vein used for IV medications, was trauma involved, was you arm pit examined, was thoracic outlet syndrome excluded, etc. Etc. So many things that only a good md could look for and detect. Before treatment is diagnosis... Se a doctor...