Weapons proficiencies are assigned at 1st level, before archetypes are chosen, and not all rogues will go on to be assassins or poisoners.
But ask your DM. Houseruling blowgun proficiency as an alternative to hand crossbow seems reasonable. Or, you could perhaps be allowed to spend downtime training in blowgun proficiency, if you can find a suitable teacher. It's not strictly within the rules because a blowgun is not a tool, but your DM can always bend the rules if it suits the story.