  We consider a Compton cloud (CC) surrounding a black hole (BH) in an
accreting black hole system, where electrons propagate with thermal and bulk
velocities.In that cloud, soft (disk) photons may be upscattered off these
energetic electrons and attain several MeV energies. They could then create
pairs due to these photon-photon interactions. In this paper, we study the
formation of the 511 keV annihilation line due to this photon-photon
interaction, which results in the creation of electron-positron pairs, followed
by the annihilation of the created positrons with the CC electrons. The
appropriate conditions of annihilation line generation take place very close to
a BH horizon within (10^3-10^4)m cm from it, where m is a BH hole mass in solar
units. As a result, the created annihilation line should be seen by the Earth
observer as a blackbody bump, or so called {reflection} bump at energies around
(511/20) (20/z) keV where z~20 is a typical gravitational red-shift experienced
by the created annihilation line photons when they emerge. This transient
feature should occur in any accreting black hole systems, Galactic or
extragalactic. Observational evidences for this feature in several galactic
black hole systems is detailed in an accompanying paper (II). An extended hard
tail of the spectrum up to 1 MeV may be also formed due to X-ray photon
up-scattering off created pairs.
