This can be verified by “purchasing”1 the Blood Hunter class on DM’s Guild.
The table in this document does not show a 6th blood curse at level 20, and matches the description of the Blood Maledict feature instead.
DM’s Guild has two different “Blood Hunter” classes; one named “Blood Hunter Class for D&D 5e (2020)” and one simply named “Blood Hunter Class”.
The latter does grant you a 6th known blood curse at level 20 and does also reflect this in the Blood Maledict feature. In this version, however, you only gain the Blood Maledict feature at level 2, while both the 2020 version and the D&D Beyond version you linked in the question grant the feature at level 1.
Therefore, we can assume that the blood curse at level 20 listed in the table is left-over from the old version, while the rest of the document has been adapted to the 2020 version (hence the link in the first section of my answer links to that version).
1 you can choose what you want to pay, i.e. “paying” 0$ is possible