Generally what is called "lilac" in Chihuahuas (the general term in dogs is Isabella) is a combination of the gene that causes "chocolate" (like a chocolate lab, with a brown nose) and the dilute gene, which causes a black dog to be blue (like your bitch, with a blue nose). My guess is that your blue bitch carries chocolate, thus the light puppy. He would be a light "lilac". Other breeds with this genetic color include Weimeraners, "silver" labs, and some Chesapeake Bay retrievers. If you look up those breeds, you will see a wide range of shades with the same genetics.
The "black" puppy is a dark blue if the parents have the genetics I described above. Dilute is recessive, so if both parents display that trait, they cannot produce black (non-dilute) puppies. See the border collie on this page. It is a very dark blue. Dog Coat Colour Genetics
Edit: Here is a link to a page that has photos of Isabella dogs in various breeds and coat types. Dog Coat Colour Genetics
There is a wide variety. Most Isabella Chihuahuas I have seen are on the darker side. Your pup is probably on the lighter side!
Look up images of "newborn weimaraner puppies" and see if some of them look like your baby. They are also super cute!
Yes, I have edited this a bunch, I am a geek about this stuff!