It really all depends! When blending for espresso, you have much more control over the many factors at hand. You can add or subtract many different components to emphasize or reduce certain characteristics of the final shot.
Single origins on the otherhand are sort of a what you see is what you get scenario. I've come across very few singles that create the absolutely perfect shot, but some have been absolute gems. For example: The crop of Natural Sidamo (gr 4) that I'm currently roasting produces a formidable shot, and is in fact one that I'd offer up as a single origin espresso!
Usually, what i've seen is that the flow and color are usually determined by the post-roast changes, like how fine the beans are, tamp, etc.
It seems that most espresso blends are a dark roast to achieve a good body and consistancy, but have minimal flavor range. The best espresso shot I have ever had was a single origin from Australia (not quite sure where in Australia) that tasted like pure honeysuckle. This was a lighter roast.
Brazil Moreninha Formosa
A few of the select Brazilian Bourbon, such as one of the estate yellow Bourbon as mentioned.
I often will roast a bean to full city or a hair longer, at say 60 percent of total, and then do 40 percent of the same bean at a city+ roast. This way you get more complexity and showcase more of what that single bean has to offer.
This works great with Brazils, it is a little trickier with the Ethiopians, but I would do it 80/20 on the Sidamo and showcase the spice, darker aspects with the 80 percent and the fruit flavors with the 20 percent.
For me, espresso should have body/good mouthfeel, complexity, persistence of flavor... from lingering sweet honey/lemon to an intense bitter chocolate. If the SO that I roast has all of these qualities, it can be as good, or better, than an espresso blend.