The subject of the paper is Anonymus' historical work on the Hungarian conquest written in the early years of the 13th century. There are two locations in the text mentioning the Black Sea; researchers were convinced until now that Anonymus refers to Pontus Euxinos in both cases, although there exists no any other source using this form for Pontus Euxinos before the second half of the 13th century, including all Western and Eastern texts. - First the expression 'ad nigrum mare' is discussed (Chapter 44; SRH. Vol. 1, page 91, lines 13-18). Identification with the Black Sea makes the text fairly confusing and must have been 'ad agaan mare' in the original. This correction eliminates all the difficulties; 'ad nigrum mare' is the consequence of erroneous copying. - The other occurrence reads as 'ad nigrum pontum' (Chapter 1; SRH. Vol. 1, page 34, lines 11-14). The earliest known version of Anonymus' sentence can be found in the so-called Justinus-epitome from the 2nd century A. D. The author presents a simplified process of evolution of this particular sentence. Practically only one rephrasing and two minor reading errors in the course of the copying process (phasi A ithasi, latere A ater, where A stands for 'arrow') help us reconstruct Anonymus' hypothetical direct source. By this reconstruction it turns out that the original 'ponto' - referring to Pontus Euxinos - has changed to 'ponto aquilonali', later into 'ater/atro ponto aquilonali', and finally Anonymus has replaced ater by niger. This model of the textual evolution not only gives an interesting example of the development of a short section of a historical work through more than one thousand years, but also eliminates a false argument for a late dating of Anonymus' work.