What kind of tree is it? How often and how much water does it get? Where do you live (area of country) and where do you keep your tree (in, out, near a south window, or an eastern one, on the TV, etc.). How warm or cold is it where the tree is?

You didn't say what kind of tree you have, so it's very difficult to give definitive advice. You're sure it's the result of too much fertilizer and not exposure to cold? Blackened leaves are often symptomatic of cold injury.

Too much fertilizer can make it difficult or impossible for your plant to absorb water (and nutrients). In extreme cases, water can actually be "pulled" osmotically from root tissues. This actually removes moisture from cells, even when roots are surrounded by water, killing tissue. The term for this is plasmolysis (fertilizer burn).

It's likely some (or much) root death has occurred if it is fertilizer burn. It's also likely that an emergency repot is in order, but I would immediately flush the soil several times with room temperature water to stop fertilizer damage. You'll probably need to remove the soil from rootage & prune back to healthy roots before you repot in fresh, bonsai soil. The plant will need to be treated essentially as a cutting while it (hopefully) recovers.

This is obviously "one size fits all" advice that may not apply to certain plants. You don't give your zone or the plant material, forcing a guess as to what you're dealing with.

Al