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Transient spectral features have been discovered in the X-ray spectra ofActive Galactic Nuclei, mostly in the 5--7 keV energy range. Severalinterpretations were proposed for the origin of these features. We examined amodel of Doppler boosted blue horns of the iron line originating from a spot ina black hole accretion disc, taking into account different approximations ofgeneral relativistic light rays and the resulting shift of energy of photons.We provide a practical formula for the blue horn energy of an intrinsicallynarrow line and assess its accuracy by comparing the approximation against anexact value, predicted under the assumption of a planar accretion disc. Themost accurate approximation provides excellent agreement with the spot orbitalradius down to the marginally stable orbit of a non-rotating black hole.Comment: Accepted for publication in A&A; 8 pages, 5 figure

The relativistic shift of narrow spectral features from black-hole accretion discs