pure damage itself goes through bkb, but most (like 75%) pure damage spells don't.
ones that do are the exception; invoker's EMP doesn't go through it, same with purification and timber chain and blood rite and pretty much all of the single target ones with the exception of impetus/doom/meat hook/one or two more maybe.
it is an oddity
invoker sucks, buff weaver 2k15
Every single spell says whether it pierces Spell Immunity. If it says "No", it doesn't damage through BKB, even if it's pure. If it says Yes, than it does as long as it's pure.
All BKB does now is give you 100% magic resistance and the state of Spell Immunity, which affects the "Pierces Spell Immunity" on every single spell in the game.
They changed it to be this way to make it less confusing, because before you had shit like "Pudge hook pulls through BKB, but doesn't damage, even though it's pure damage." or "Primal Roar goes through BKB, but the damage does not."