Thanks to its high Kerr non-linearity and its low linear absorption, silicon is a material of choice for optical devices in the mid-infrared (from 3 to 5 microns) such as microresonators. In this wavelength range, the available optical sources such as quantum cascade lasers have a limited tunability. Tuning the refractive index of silicon can be achieved by a temperature change of the chip and has been previously demonstrated on ring resonators using integrated heaters or thermo-electric elements. We present a new method for thermo-optical tuning of silicon devices by directly using the light from a laser diode operating at 450 nm. The blue light focused on the silicon induces a local elevation of temperature and thus the refractive index locally increases. When applying this method on silicon ring resonator, the elevation of temperature leads to a decreasing free-spectral range and thus shift the resonances to lower frequencies. At 4.5 µm we measured a tuning efficiency of 200 MHz per mW of incident light. Numerical simulations of the thermo-optical effect show the locality of this tuning method, and confirm the experimental results. Finally a frequency study of the response of this method is performed and a time constant of the order of the micro-second is measured. In conclusion, we propose a fast, local, and non-invasive method for tuning silicon resonators operating in the mid-infrared that can be extended to any silicon-based device.