We report on Gemini, NuSTAR and 8-year Fermi observations of the most distant blazar QSO~J0906$+$6930 ($z=5.48$). We construct a broadband spectral energy distribution (SED) and model the SED using a synchro-Compton model. The measurements find a $sim 4 times 10^9 M_odot$ mass for the black hole and a spectral break at $sim$4 keV in the combined fit of the new NuSTAR and archival Chandra data. The SED fitting constrains the bulk Doppler factor $delta$ of the jet to $9^{+2.5}_{-3}$ for QSO~J0906$+$6930. Similar, but weaker $delta$ constraints are derived from SED modeling of the three other claimed $z>5$ blazars. Together, these extrapolate to $sim620$ similar sources, fully 20% of the optically bright, high mass AGN expected at $5<z<5.5$. This has interesting implications for the early growth of massive black holes.
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