Small particles such as youve described have no effect on the image projected onto the film. If you have small particles of opaque black material inside, those are most likely flakes of the dull, black light-absorbing coating applied to the inside of the lens barrel and are of no consequence, not even when photographing a light colored area.
If youre uncomfortable with the particles inside the lens and can return it, you should do so and find one thats satisfactory.
We discussed this about 2009 or 2010 or so in a thread entitled something like Black Specs on Lens. Unfortunately, due to the ongoing server issues that thread isnt currently accessible so I cant provide a link at this time.