I just recently completed a 2 week series of classes given by Tim Holtz, and 1 class was dedicated to the Distress Markers.
They are very blendable on all kinds of surfaces including watercolor paper, glossy paper, stamping paper etc.
The markers are water based dye inks, so you can't blend the the colors like you would with a Copic marker.
Distress markers are blendable with water. They, like the rest of the distress line of products, are reactive with water. If you lay two colors down next to each other, you can pull the colors and blend them using a water brush.
You can also blend them by coloring with a light color, add a darker color to the edges, go over the darker color again with the light color, then pull the color in with a water brush to create a blended/shaded look.
Another way to blend the colors is to lay down color from one marker on a non stick surface, then use another marker to pick that color up off the surface and apply it to your project that way.
Oh--and, if you want to use them with a stamped image, you'll want to stamp with an oil based &/or permanent ink otherwise your image will smear when working with water.
The Picket Fence marker is a whole other animal.
He also mentioned that they can be stored upright or sideways.
Head to Rangers website for a video on these markers, but I'm sure there are a gazillion others out there, too.
Hope this helps!!