I personally don't think blood magic is naturally corruptive... it just tends to end up this way in practice. But that ties to my idea of schools of magic (as presented in DA:O) being more than just a way of classifying spells - but also philosophies of using magical power. As we know, a mage has limited resources, so he must study magic to optimize his use of power and focus he has. And the four basic ways mages in Thedas do it are as follows*:
1. Spirit magic: Spellcasting is about drawing energy from the Fade and shaping it with your mind; therefore you must study Fade and learn how mind works - those are your strenghts (this school specializes in Fade- and spirit-related stuff as well as with invisible "pure" magical force; anti- and metamagic goes there too)
2. Primal: Physical world has its own powers, it's best to make use of them rather than work against them. You must study those forces and learn how to make them act, they'll do the rest like an avalanche brought down by a single pebble (this school favors elemental powers but often lacks control since magic is more of a "trigger" and unleashed energies are hardly controlled)
3. Creation magic studies physical objects, and living things, empowering, changing and shaping, sometimes even creating by just filling pattern with substance. It's more concerned with material patterns than energy itself.
4. Entropy resembles Creation a lot, but when Creation is interested with Whole and gently filling up cracks and/or making things as they should be, the entropy is about analyzing patterns, finding weak spots and throwing in some pebbles (your magic) in carefully chosen places of complicated machinery of existance to make it jam/weaken/fall apart
Those are the four schools, but there is a fifth one, opposed to all three in the basic concept - while they are about doing what best you can within your limits, perhaps extending them with practice, Blood Magic is different. It doesn't ask "how do I best utilize what little power I have", it asks "how do I get more power, reach beyond my limits". It can do things no other school can, it lets you draw power normally impossible to obtain save with significant quantities of toxic lyrium... But it has a price. And, as Dorian says - there is no harm with using your blood or that of willing participant... but what if you want more?
And wanting, needing and reaching for more power is in the very philosophical core of Blood Magic. When you start studying it - when do you stop? What if you get really desperate, suddenly?
Blood magic isn't evil, it's just a tool, but a poisoned dagger isn't evil either. There certainly are people who could use BM responsibly without succumbing to darkness - the problem is, however that those are people who don't feel that they really need power. People who are willing to say stop at some point and accept the limit, regardless of consequences of not being powerful enough. The only problem is that those are precisely not the people who would seek out BM, especially when it is forbiden and especially especially if they would need to make a deal with a demon to learn it.
Therefore BM tends to end up in hands of power-hungry and desperate, which translates to: people who should never have ability to sacrifice lives for their power. And that's where BM's bad reputation comes from: it's most attractive for exactly the king of people who should never learn it as they are pretty much guaranteed to get corrupted... not by Blood Magic itself, but by their own desire for power.
And then there ae all those terrible things you can actually do with BM - things you don't really want to see in hands of power-hungry or truly desperate, now do you...
*All this is my personal head-canonish speculation, it doesn't come directly from any official soutrce