Should I put the blue light on my air conditioner to remove and prevent mold. Does it really work or is it a scam?

Blue or untraviolet tube? Many air filtration systems and furnaces use an ultraviolet light and it does kill bacteria but we can't tell exactly what you bought. Can you tell us the brand or model?

I was told to purchase an ultraviolet blue light that is installed in the register of the air conditioning unit and that it would (in time) clean up the old mold and prevent new mold from collecting. As I understand it, there are a number of these units for sale. Mine will cost $250 installed and I do not know yet what brand it is. Does that matter?

Mold forms when there is repeated dampness on something. Where is your dampness coming from? An ultraviolet light will kill air bourne bacteria but is not so effective in killing a cluster of mold inside your unit. Is the mold in the home or in the ducts?