This issue came up recently in a game I'm running, and I wanted to throw in my two cents.
I hold the unpopular opinion that dark-vision DOES indeed negate Shadow Blend, for the following reasons.
1. All the arguments FOR Shadowblend>Darkvision hinge on several extrapolations of the rules, and nothing that is actually printed in the book itself. The "Su" trumps "Ex" argument is pure extrapolation/opinion, and has no real basis other than "This is the way it makes the most sense to me". Someone could theorycraft argue that all day without either side making any real headway, but for the purposes of my post, point 1 is that NO WHERE in the material does it say that "Su" trumps "Ex". Therefore, that argument is invalid.
2. Next, for those of you who still believe that Shadowblend, being a supernatural, should trump darkvision, I'd like to take to an argument based out of the "Race Builder" in the Advance Race guide. If we look at the "Shadow Blending" trait in the race guide, we see that it is only world 1RP, whereas Darkvision, the ability that supposedly cannot counter it, is worth 2RP. The BOOK ITSELF seems to be telling us that Darkvision is, by and large, a stronger ability than Shadow Blending. On top of this, Shadow Blending is down at the bottom of the trait pool as one of the cheapest traits in the game, even though it's proposed usage would be extremely strong. I think the book itself makes it clear that the authors did not intend for Shadow Blending to work against Darkvision. For those of you who prefer to make arguments based on extrapolation instead of actual rules in the book, I'd love to see a counter to this, combined with my next point.
3. Let's now take a look at the actual rules of the game. Looking at Fetchlings in the Advance Race Guide, the description of Shadowblend is as follows:
"Attacks against a fetchling in dim light have a 50% miss chance instead of the normal 20% miss chance. This ability does not grant total concealment; it just increases the miss chance."
The part of this that I want to focus on is this bit: "50% miss chance instead of the normal 20% miss chance. This ability does not grant total concealment". Given what this says, the ONLY information we have from the book is to then look at how dim light affects concealment. In the Additional Rules section, we glean the following information about Dim-Light concealment:
"In an area of dim light, a character can see somewhat. Creatures within this area have concealment (20% miss chance in combat) from those WITHOUT darkvision or the ability to see in darkness"
This is THE most important, most rule-based argument on the subject. The rules CLEARLY state that darkvision negates the 20% miss chance that the 50% miss chance for Shadow Blending is based on. The skill itself even states that the skill is SIMPLY an extension of the original miss chance. It doesn't change what does and doesn't negate the miss chance. It doesn't magically merge your character into a shadow so that a darkvision character looks right past you. It ONLY adds to the 20% concealment chance in dim light.
So. That's my argument. I feel that there is no RULE BASED way to beat the argument I've presented, so I welcome you guys to try, although I will say, I'm going to completely ignore any argument that is based on personal extrapolation, for reasons stated above.