This is because Fehling's solution is a mixture of two solutions:

* Fehling's A: A solution of copper(II) sulfate, which is blue.

* Fehling's B: A solution of potassium sodium tartrate (Rochelle salt) and sodium hydroxide, which is colorless.

When these two solutions are mixed, the copper(II) ions from Fehling's A react with the tartrate ions from Fehling's B to form a complex ion. This complex ion is responsible for the characteristic deep blue color of Fehling's solution.