  We have selected SDSS J222032.50+002537.5 and SDSS J142048.01+120545.9 as
best blazar candidates out of a complete sample of extremely radio-loud quasars
at z>4, with highly massive black holes. We observed them and a third
serendipitous candidate with similar features (PMN J2134-0419) in the X-rays
with the Swift/XRT telescope, to confirm their blazar nature. We observed
strong and hard X-ray fluxes (i.e. $\alpha_X<0.6$, where
$F(\nu)\propto\nu^{-\alpha_X}$ in the 0.3-10keV observed energy range, ~1-40keV
rest frame) in all three cases. This allowed us to classify our candidates as
real blazars, being characterized by large Lorentz factors (~13) and very small
viewing angles (~3deg). All three sources have black hole masses exceeding
10^9Msun and their classification provides intriguing constraints on
supermassive black hole formation and evolution models. We confirm our earlier
suggestion that there are different formation epochs of extremely massive black
holes hosted in jetted (z~4) and non-jetted systems (z~2.5).
