I'm not a avid league player, but as far as I know Flash has been mandatory in competitive on like 99% of champs for pretty much most of leagues existence.

Blink is just a different approach on the topic of instant movement skills.

Flash is free and has no downsides, but is rather short and has a long ass cooldown.

Blink Dagger is fairly expensive, can be disabled through player damage, gives zero stats but has long range and a reasonably short cooldown.

Both fundamentally shape their game for the better, because without them the game would be a lot slower and less spectacular. Probably 80-90% of all "greatest highlights of dota" scenes feature a blink and I presume it's vice versa in league.