The author examines the passage in the little Old Church Slavonic treatise On the letters by Monk Khrabr in which are mentioned the margrave Kocel’ and, above all, the place of his ‘residence’, Blatьnьskъ kostelъ. More specifically, the author intends to prove the groundlessness of the arguments proposed in a recent article by M. Capaldo who refutes the genuineness of the reading (kôcelå knåza) blatTMnska kôstelå, transmitted by the ms. Mountain Athos, Hilandar 481. In addition to the intrinsic authenticity of the toponym Blatьnьskъ kostelъ – which corresponds both to the Old High German Mosapurc and to the Latin urbs paludarum, as noted by R. Nahtigal –, the reading of the Athonite manuscript proves to be the original since it is the only one capable of explaining the genesis of the other variants attested in the rest of the tradition. Therefore there is no valid reason to doubt the genuineness of the above-mentioned reading, whose presence in the ms. Hilandar 481 is due to a particular form of contamination, which explains also other readings characteristic of that witness.