as per page 74 of the basic rulebook:

Reload:

A weapon with the reload quality requires extra time to load and fire. The character must perform a special reload manoeuvre
with this weapon before using it for an action or else the action check suffers an additional <P> challenge die.

Am I correct in assuming that this "special reload manoeuvre" is just a manoeuvre? This sounds a bit easy for me... Now a blunderbuss could essentially be used every turn, whereas in 2nd Ed. you had to spend 3 turns reloading it. Am I missing something or is this just a thing for houseruling it a bit?

It uses up a standard maneuver to Reload. A character can shoot every round without "reloading", they just suffer the additional <P> penalty. So, unlike most games, part of the shooting action is also loading/reloading it hastily. The maneuver action means the character is taking their time to ensure that the blackpowder weapon is loaded/reloaded carefully and properly.

Bear in mind that black powder weapons have Unreliable 1 or 2 and so the additional chance of a chaos star is a big risk.

Indeed, but normally a range-weapon fighter does not want to use his free maneuver for anything else (at least it seldomly happens in our group, as they are always carefully adjusting their tactics). So He/ she can "sit" there, firing his/ her blunderbuss every round without extra purple dice.

In WFRP2 the 2 rounds of reloading were a nuisance for our dwarf coachman, so he just opened up the fight with one shot, then slang it back onto his back and drew his axe to engage in melee.

Also keep in mind that the actual length of a round in WFRP 3 is more variable then before. It's not a case of a round is 6 seconds.