Black Bolt communicates via sign language. An idea was proposed to have a voiceover from Anson Mount as telepathy, but the filmmakers ruled it out, not only to make Black Bolt more enigmatic but to appeal to deaf-mute viewers.
This was most generally considered Marvel's biggest "risk" as the Inhumans were not nearly as well-known or popular as the X-Men or the Avengers, which was why Marvel's Marvel's Agents of S.H.I.E.L.D. (2013) spent its entire second and third seasons focused on developing and establishing the Inhumans in the MCU, several years before the debut of the Inhuman Royal Family.
The Inhumans are a 7,000-year-old group of early humans that were experimented on and given powers by the Kree. The Kree previously appeared in the Marvel's Cinematic Universe, in Guardians of the Galaxy (2014) and Marvel's Agents of S.H.I.E.L.D. (2013).
When originally conceived as a film, Inhumans (2017) was to be Chapter 11 of Phase 3 in the Marvel Cinematic Universe before being removed from the schedule. However, a film centering on the Royal Family may still happen in the future.
Was originally announced on October 28, 2014, as a film along with Black Panther (2018), Captain Marvel (2019), and Avengers 3 & 4 (Infinity War & End Game).