please help

Hi, I am a new chihuahua owner. Rosie is my first chihuahua I have ever had. Recently I have noticed she has has these black dots under her skin. They can be pulled out and it does not hurt her. As well her skin around her legs is turning black. And just this week she has started to smell...

So right now I feed my dogs Wellness CORE and Kibbles N Bits, I want to completely move them onto feeding them Merrick Grain Free mixed with Wellness CORE. Merrick Grain Free Real Texas Beef & Sweet Potato Dog Food at PETCO Has anyone heard of Merrick before? Or tried it? Opinions on my idea...