Vokun ViingYep, but Dragons appearance is happening in Akavir in Lore.
According to your logic, we don't know anything. You think so because it's the myth, created but man's fiction and it can be changed any time.
Akaviri race Tsaesci enslaved Red Dragons, but Black Dragons fled, as the Lore says, (but after then, war has begun and all of the remaining Dragons were killed. Then Tosh Raka became the largest Dragon in the world. So, Dragons popularity in Akavir isn't ended by that time, so Akavir Dragons can make return any time)) during that time, dragons in Vvardenfell were overrun by cliff racers, may be thats the time when they, black dragons reached Skyrim? We don't know exactly, as you say, and we'll get enough knowledge about that soon