What element is needed to turn Hydrangeas blue? Is it sulfur or aluminum?
You want to acidify your soil, so sulfur because that is used to make sulfuric acid.
If you just want blue hydrangeas (Hydrangea macrophylla only) in pots, then a dose of aluminium sulfate will do the trick quite quickly. Either that or sequestered iron will do it, but in the ground, it's harder, because substances applied to the soil will only have a temporary effect - sulfur will need to be added frequently, and it takes a long time to make a difference; frequent use of aluminium sulfate is not recommended - used too often, aluminium can build up in the soil and phosphorus levels can be reduced. More general information re acidifying soil here https://www.rhs.org.uk/advice/profile?PID=82