Someone comment?

A lot more information is needed to help. What is a Geranium 'tree', where is it growing and can you provide a picture?

Look for aphids. The aphids excrete honeydew as they chew. The "soot' is a fungus that grows on the honeydew. Try just hosing off your plant with a strong stream of water to dislodge the aphids.

The "soot" can sometimes be wiped off but I would bag the affected leaves and dispose of them.

Did you train your Pelargonium as a standard yourself?