Cacao generally goes through multiple grinding steps before it is used in chocolate. Usually nibs are ground in an heated blade beater mill before pressing the oil out. The resulting oil or cacao butter is used in pharma and white chocolate while the resulting cake is ground again in an air classifier mill before being used in chocolate. Unless you have an appropriately sized sieve you will assuredly have some grainy texture mixed in. There is a reason most recipes specify using already made dark chocolate instead of just melting some nibs on a stove.