Roma were fined 50,000 euros (£42,000) after Sunday's match was suspended for two minutes shortly after the half-time interval by the referee.
Blatter said he could not understand the leniency of the fine and warned that FIFA's Congress would agree much tougher sanctions for racism at the end of the month.
Blatter told FIFA's website: "What is most surprising and is not understandable for me, is that the disciplinary committee of the Italian Football Federation has taken a decision, not even 24 hours after the event, by just imposing a fine.
"They have not made any investigation of what happened. And just to give a pecuniary sanction is not valid, that is not acceptable.
"You will always find money. What is 50,000 euros for such an incident? I'm not happy and I will call the Italian Federation. That's not a way to deal with such matters."
The referee, Gianluca Rocchi, suspended the match to allow the stadium announcer to order offenders to desist before play was allowed to resume.
That procedure was introduced in the wake of Milan midfielder Kevin-Prince Boateng's decision to lead his team-mates off the pitch during a friendly game at Pro Patria in January, forcing the abandonment of the match and sparking new debate on the subject of racism in Italian football.
Blatter said he was stunned that racism had again raised its head in Italy.
He added: "I think lessons have not been learned. It is incredible, that we had such incidents especially in the Italian Serie A in the San Siro between AC Milan and Roma, a very important match.
"The referee had to stop the match for a few minutes in order to bring back calm. This is bad, because we are just holding this task force against racism.
In two weeks, we have our FIFA Congress, and we will have a resolution, that will be binding for everybody.
"These sanctions must be applied all around the world. That's why we need the Congress's decision. That will bind together all the 209 associations."