Blazars are radio-loud (RL) active galactic nuclei (AGNs) with relativistic jets pointing towards the observer. Since their non-thermal radiation is relativistically amplified, and not obscured along the jet direction, blazars are very bright and visible up to high redshifts. The discovery of high-z blazars ensures the census, free from obscuration effects, of supermassive black holes (SMBHs) hosted by RL AGNs in the early Universe, and provides constraints on the accretion mode, mass and spin of the seed of SMBHs. Indeed for each observed blazar we expect to find ~2Γ^2 (where Γ is the bulk Lorentz factor of the jet plasma) misaligned RL AGNs, whose jets point in other directions with the same characteristics (e.g. radio power, black hole mass, etc.). Therefore the selection of blazar samples at high-z suitable for statistical studies and the discovery of more blazars at the highest redshift (until z=6) represent the two best ways to study the RL AGN population in the early Universe. Selecting high-z blazar candidates requires a multi-wavelength approach and the use of large area surveys that cover almost the total of the entire sky at a reasonable depth. In particular, we are carrying out a project which combines optical, IR, and radio datasets to identify blazars at redshift z>4.0, pushing the search into a range of redshift (z≥5.5) not yet explored for this type of AGNs, by also using the so-called dropout technique. First of all we selected a statistical radio flux limited sample of blazars at 4.05, coming to find out the first blazar at z>6 (PSO J0309+27).