[zzzzz]

where does the T from the indicator Eriochrome Black T come from?

[Borek]

Therea er at least four Eriochrome blacks - A, B, R and T. They are very similar compounds. all have analytical uses. Capital letters are used to differentiate between them; perhaps they are connected with the compund structure but I am only guessing here.

[ATMyller]

Eriochrome black T is a compelx aromatic compound used as a metal indicator, printing pigment and in water hardness analysis.
It's blue in protonated form and red in complex form. It's known for ages so it's probably extracted from some plant.