Police are due to interview a number of other people, including the ball boy who claimed the Senegal international made a racist comment to him during the 3-0 defeat at Goodison Park.
Diouf was interviewed by police, accompanied by a Blackburn official, in a private room an hour after the match.
The Senegal international was seemingly involved in an altercation with the ball boy during the first half.
Diouf then suffered abuse from home fans, and again when he went to take two corners in the area of the original incident.
A police spokesman said: "Merseyside Police are looking into an allegation that a 28-year-old man used racist language at the Everton match."
The ball boy lodged his complaint with his ground supervisor, who in turn told Everton officials, who then informed the police.
Blackburn manager Sam Allardyce was not made aware of the incident until after his post-match press conference yesterday.
However, it is believed he spoke to Diouf on Monday morning as the players were brought into training early to review the defeat, with Allardyce due to attend the Sir Bobby Robson memorial service later in the day.