The term “power creep” means pretty much what it sounds like. The idea is that games tend to accidentally create better, more efficient content as release schedules continue and cause existing content to lose value. In a CCG’s case, this means making cards that are strictly better than existing ones. There’s a YouTube channel I’m very fond of called “extra credits.” They have an episode worth watching that can explain it to you further.
Looking at blood of air, people compare it to both dark transformation and entropic decay. It’s the same cost as transformation but spawns a more powerful minion which has rush, however this minion is temporary and the card belongs to a different faction. As for decay, blood of air costs 1 more mana but has range and spawns a minion.
I’m both cases, I’d agree that this card is more powerful. However, in neither case is blood of air strictly better. For something to be strictly better, it must not have any drawbacks compared to the prexisting content (for example, if it were 4 mana, it would completely invalidate entropic decay).