We provide first observation of the effect of spatially uniform temperature change, causing non-affine thermal strain, on the optical properties of amorphous Si_xGe_1-x(x = 0.85, 0.60 and 0.15) thin films. The thermal strain was introduced by 7 cycles of cooling from room temperature to 77 K. It led to a decrease of the transmission, increase of absorption and red shift in the absorption edge (darkening). This effect of cycling is similar to the effect of increasing the Ge content. Analysis of the XPS spectra has shown that a certain fraction of homopolar Ge-Ge bonds were broken and Si-Ge heteropolar bonds were formed with cooling cycles. These changes, as creation of extra defects in the amorphous structure, can cause darkening, similarly to photo-darkening induced by laser irradiation. The darkening via thermal strain is a suitable technique to tune the optical and structural properties of thin films for different applications.