Using a Vitamix-style blender is going to give you the best results, producing the smoothest & creamiest texture. However a regular blender will work as well - leaving only the slightest noticeable texture.
Citric Acid is the sourness that's found in lemons. It impart tartness to foods, without adding any particular flavor. It also is a natural preservative and color enhancer - helping to maintaining the ketchup's beautiful red color.