This chapter will probably be never addressed anymore. We have to look up from the sources we have, like:
https://wowpedia.fandom.com/wiki/Blood_Knights
there you have a line:
Although Warchief Thrall and High Chieftain Cairne Bloodhoof openly opposed the blood elves' methods, they recognized the Blood Knights' strategic value. Indeed, it is likely that the group figured heavily in the Horde's ultimate decision to offer membership to the blood elves.
This alone points out to the fact that Blood Knights were already established group and were one of the main reasons for Thrall to consider inviting blood elves to the Horde. That leads to the fact that draenei were already on Azeroth. When Kael attacked Tempest Keep and captured M'uru, draenei fled with Exodar and arrived to Azeroth. If blood elves would be part of the Horde prior to the draenei arrival, that means they would not have M'uru in their captivity, and thus no blood knights, which is direct contradiction with both quote from official source and in game content.
Exodar is dimensional ship, which can travel trough space and time.
https://wowpedia.fandom.com/wiki/Dimensional_ship
Dimensional ships (or trans-dimensional ships) are vessels created and operated by the naaru that can travel between worlds and dimensions, traversing the Twisting Nether just as normal ships traverse the air and sea, and warp time itself.
Sure, it probably took some time for draenei to reach Azeroth, but probably not really long, because blood elves infiltrated the ship and sabotaged, so it crashed. Also, we've seen other ship, Vindicaar, even before its empowerment, being able to reach Argus in a matter moments. In fact, when you do quests in Azuremyst, it is not stated how long it is after Tempest Keep is lost, but most of the draenei are still shaken by that and by the crash, of course.
Also, founding of blood knight order probably took some time, as well as their training and growing in prominence. As I pointed out above, blood knights were important factor for inviting blood elves to the Horde.