Our answer is a guarded yes for mayonnaise and "not recommended" for hollandaise sauce. Although blender mayonnaise cannot rival the hand-whisked variety, it can be good if properly made. Blenders are slightly superior to food processors for making sauces like mayonnaise because their thinner, sharper blades produce a smoother texture and incorporate more air into the developing sauce.
Some cookbooks have recipes for blender or food processor hollandaise sauce. If you follow these recipes, the egg yolk will be insufficiently cooked and the sauce's texture will lack the silkiness of authentic hollandaise.
** Asian Recipes