Well, it is all about liberating the phytonutrients from the rigid plant cells. You can do that by cooking (at the cost of loosing some heat-sensitive compounds) or by mechanical means - chewing thoroughly or much better, putting it into a high-performance blender. Cooking the vegetables prior to blending them, wouldn't make much sense. You'd only loose the heat-sensitive compounds and compromise the taste without significantly enhancing nutrient absorption.
Many phytonutrients (such as carotenoids) are fat-soluble and the presence of some fat significantly increases their absorption. Pepper also increases absorption of many (not only beneficial!) compounds by inhibiting P-glycoprotein (which inhibits the uptake of non-nutritive substances by pumping it out of the cell, back into the intestinal lumen). So you better make sure not to have any toxic herbs in your smoothie when adding pepper to it 
[Regarding references: Don't be lazy. There are enough to find in this forum and on the web. Use google.]
Fiber may help to boost absorption due to increased transit time, but as long as you use only whole fruits and vegetables for you smoothie (instead of juices) it should already provide more than enough fiber, so there is need to add any further fiber to it.
At this time of the year, I regularly make smoothies from wild greens. They are about 1 part greens, 1 part fruits and 1 part water by weight (plus some ginger, turmeric, olive oil and pepper). The wild greens provide so much fiber that I have to warn anyone tasting it not to drink more than a small glass at a time, because I have seen people not accustomed to a high fiber diet rushing to the toilet after drinking too much of it...
Edited by timar, 29 April 2016 - 08:57 AM.