Carmen Rios Garcia, 23, is one of seven people in custody in connection with the crime, an official told EFE.
"What we think - it's a hypothesis - is that it was a satanic ritual. They speak of a religious ceremony," Mayra Perez said.
The incident occurred at 8.30 a.m. in the town of San Agustin Atlapulco, where 10 people, three of them minors, were inside a house performing "a religious ceremony", she said.
One of those present, the brother of Rios Garcia, told the authorities that the woman "had the child in her arms" and asked the other attendees at the ceremony to close their eyes".
It was then that Rios Garcia stuck her fingers into her son's eyes, blinding him, claimed Perez.
Alerted by relatives of the boy, police transported young Fernando to a nearby hospital, from where he was ultimately transferred to a better-equipped facility in Mexico City.
According to the boy's uncle, Jesus Rios, several of the people present at the ceremony, which lasted several days, were under the influence of "some drug or in shock", a condition that prevented them from helping the youngster.
The Mexico state Attorney General's Office opened an investigation into the incident.
Besides the people arrested, a child of 8 and a 9-month-old baby were rescued from the home where the ritual occurred.
Regarding Fernando's condition, officials at the Mexico City health department said that he is under sedation and out of mortal danger, but he sustained severe trauma - and irreversible damage - to his eyes, and thus he will remain blind for life.