We have proposed a method for calculating the uncertainties arising when using a boundary condition of the third kind in characteristic sections of the profile of a cooled gas turbine blade (flat smooth wall, ribbed wall, and wall with an air curtain). It is shown that uncertainty for a smooth wall increases with increasing values of the Biot numbers, while the error can reach 1.47 % with Biot numbers equal to 1.0. For a ribbed wall, the uncertainty rises to 2.4 %. The largest error associated with application of a boundary condition of the third kind occurs on a plate with a curtain blowing (the uncertainty can increase to 11 %). This method has been validated using the example of a cooled working blade at the Central Institute of Aviation Motors. Applying boundary conditions of the third kind introduces a significant error in determining the thermal state of the blade with a convective-film cooling system (especially for concave surface of the blade). The difference between the approaches is less pronounced on the back of the blade, due to formation of a more stable cooler film. Accounting for uncertainty allowed to reduce the error in calculating the thermal state of the cooled blade from 8 % to 3 %