As the noob said - blighter takes 5 levels of Druid and turns them into medium BAB, skill points, and saving throws. You lose your spellcasting, wildshaping, and animal companion. In exchange, you get spellcasting that doesn't even catch up to druid casting levels until level 10 (druid5/blighter10 has 5th level spells) and NEVER catches up in spells/day. It also draws from a VERY limited list of spells known. Similarly, you gain a limited form of wildshape that's pretty much worse than druid wildshaping at all levels (your blighter levels don't stack, so you never really can shape into high-hitdie monsters). The undead template is kind of cute and replaces some of the things you're missing from plant wildshape, but in the end, blighter wildshape isn't all that hot. You also never get your animal companion back, and we know how good that is.
And then, if you last to level 15, you're hosed. Where do you go next? You can't go back to druid, you can't advance your casting or anything. You're just stuck. It's like the opposite of Ur Priest, which you can get into using a non-cleric entry, and can be advanced nicely with other dual-progression classes. Blighters just stink.
Shining Blades stink, but they stink because even for a paladin, they don't add a whole lot of power to your build. But Blighter is the only PrC that I know of where levels in it actually make you weaker. If you had to break your Paladin vows to become a shining blade, that would stink on this level.