I noticed in your recipe for Salade Niçoise,that you say to put all of the dressing ingredients into a min-blender and whiz until smooth. I thought when making a vinaigrette or mayonnaise that you had to drizzle the oil in a drop or two at a time!
When making a vinaigrette or mayonnaise by hand you need to drizzle the oil in slowly while stirring constantly in order to make an emulsion that doesn't break. An emulsion is a homogenous mixture of two or more liquids that normally don't usually mix together, in this case oil and water. This is achieved by breaking one of the liquids into very small drops in the other liquid. To keep the drops from separating back out, you need an emulsifier. Egg yolks, mustard and honey are all common emulsifiers that can be found in most pantries. Some health food or specialty stores sell soy lecithin which also works.
To understand how an emulsifier works, imagine two villages. One village, lets call them "O", will only shake hands with their right hand. The other village, "W", will only shake with their left hands, so "O" and "W" can never get along. They never trade, or make friends because they can never shake hands on anything. The people from 'the other village' are just strange, so they never mingle.
One day a bunch of people from "E" comes along. They can shake hands either left-handed or right-handed. By shaking with "O" with their right and "W" with their left, they broker peace and friendship.
OK, a little far fetched, but that is kind of how emulsifiers work. They are molecules where one end likes to hang around with oil and the other end likes to hang around with water. Once the oil in your vinaigrette or mayo is broken into small droplets, they get coated by emulsifier molecules that keep them from separating from the surrounding water, and forming back into big globs that will rise to the surface.
The reason for pouring the oil slowly in the traditional hand method is to have as much of the oil as possible in small drops. If you don't, then the small drops will bump into the bigger drops and merge with them, making bigger and bigger puddles that will make the dressing separate.
When you use a blender, the blade of the blender chops the oil into tiny drops faster than you could ever achieve by hand. I have seen lots of recipes for blender sauces that still insist on drizzling the oil in, but really there is no need. Put everything in, crank it on high for a few seconds and you are done. I almost always use this method, unless I'm showing off making a Caesar Salad, with no trouble. Occasionally, a few drops of oil may surface if I make the vinaigrette a long time in advance, but another quick buzz is all it takes ot solve the problem.
The only exception is if you are making blender hollandaise or anything like it, that uses egg yolks and melted butter. In that case, if you just throw everything into the blender, the protein in the egg yolks may scramble from the heat of the butter before the sauce is blended. Drizzling the melted butter in will temper the eggs while the sauce is being made.