  Blazars are Active Galactic Nuclei characterized by relativistic jets
launched in the vicinity of the central engine (i.e. a supermassive black
hole), that are oriented close to our line of sight. Their peculiar orientation
makes them very efficient tracers of the overall jetted population, and due to
their brightness they can be visible up to very high redshifts. A deep
knowledge of these objects can provide fundamental clues to the models of
formation and growth of the first supermassive black holes, but their search in
the early Universe must be careful and follow a systematic approach. The
discovery in the last $\sim15$ years of extremely massive blazars at very high
redshifts ($M_{\rm BH}>10^9M_\odot, \, z>4$) revolutionized our perception of
their earliest evolution: there seem to be different formation epochs for
extremely massive black holes hosted in jetted ($z\sim4$) and non-jetted
systems ($z\sim2.5$). This is not easy to explain, since one would expect that
jetted sources accrete less efficiently. Small differences in the population
are also derived from the search of such high-$z$ sources: we will go through
the open questions in order to understand where the common knowledge stands and
which steps must be undertaken to better understand the formation and common
evolution of supermassive black holes and jets in the early Universe.
