Evolve is a completely different card with a completely different mechanic.
Basically, evolve gives much more health and also attack damage. It also has a chance of giving minions with good effects.
Blood imp has a lower impact than evolve and also has more risk since its very sceptical to aoe.
What I am saying is, evolve has the "give 1 hp" mechanic of blood imp but it's on steroids.
Can't compare them in a vaccum because they belong in two different classes. If blood imp read the way it used to in the current version of zoo lock it would really really good. Warlocks ability to flood the board is just better than shamans. Previously because of card draw, now because of card draw and Forbidden ritual. That combined with something that puts a lot of those minions out of range of easy removal would be a nightmare.
Shaman does have an easier time creating many minions on the board now than it used to but still runs out of steam faster than lock because of card draw. The overload mechanic also slows them down as well. Also there are a lot of subpar drops you could get from evolve. Sure it gives bonus stats but it could also give you something out right bad like a doomsayer out of a totem, or it could give you a horribly understated minion due to a battle cry or the like. Meanwhile the warlock gets exactly the minion they meant to play, only tougher.
I am not arguing that evolve is bad, it's actually amazing in shaman. Getting some stat upgrades for your totems and what not. Just that the way shaman works vs how warlock works keeps the scales more or less balanced.
Personally I find the thought of the original original blood imp to be far scarier than evolve in the current, still evolving meta. Both Warlock and Shaman have face decks right now and find shaman to be less intimidating than warlock. Just my opinion of course. At least with most shaman face lists Evolve the card doesn't see much play (although Master of Evolution does) and their board generally stay cleared more easily. I can't keep a lock board clear for crap.
It really is a lot different though. Evolve is a card that you have to combo with a full board and it definitely has a way bigger impact than Blood Imp on the turn it's played. On the other hand, the old Blood Imp is great because you can just drop him on turn one and he will continue to buff your board until your opponent can clear him.
They're played in an entirely different context so you're comparing apples to oranges. Evolve is a much more powerful card but it's also a lot more situational to get value out of and infinitely more RNG dependent, which is why it's impossible to say which one is stronger.
Evolve is definitely undercosted. Easily a 3-4 mana card at the minimum. The ability to instantly upgrade your board is just too good.
It doesn't just upgrade your board; you can trade in such a way that you leave all your minions highly damaged, then evolve them up to fully healed, more expensive minions. The potential of that single one mana card is freaking absurd.