It can be yellowish, but you need to be able to clearly see right through it, with no speckles or clouds, or you might get stearic spots. Which are only a problem if you don't like white spots in your soap.
Also, do you let the air out of the stick-blender bell before you begin blending? Depending on the design of the bell, it may not be possible. If it isn't possible to let the air out of the bell with your current blender, you'll have to accept air bubbles as part of your design, switch to stirring by hand, or get a new stick-blender that you can let the air out of.