The Senegal international is believed to have complained that the ball was not thrown back into play quickly enough.
Diouf then suffered abuse from home fans and again when he went to take two corners in the area of the original incident.
After the match police interviewed Diouf at the stadium before the Blackburn team left.
A spokesman for Merseyside Police said: "We are looking into an allegation that a 28 year-old man used racist language at the Everton match today."
It is not the first time Diouf has found himself in hot water for his behaviour during a match.
In 2003 he was banned for two matches after being caught on television spitting towards Celtic fans while playing for Liverpool in a UEFA Cup quarter-final at Parkhead.
The former African Footballer of the Year was also banned for three international matches in 2004 after being found guilty of violent conduct in an African Nations Cup quarter-final with Tunisia.
Later the same year, now playing at Bolton, he was accused of spitting in the face of Portsmouth captain Arjan de Zeeuw and was fined two weeks' wages by his club.