Karmelo Anthony allegedly sent his then-girlfriend a photo of a knife and talked about stabbing someone shortly before he killed Austin Metcalf ... according to evidence jurors never heard.

The alleged messages surfaced Thursday during a hearing on Karmelo's push for a new trial, according to multiple reports from Dallas-area outlets.

Prosecutors say Karmelo's girlfriend told a school administrator she was scared of him ... and Karmelo allegedly later texted her a knife photo and wrote, "I'm lowkey on the verge."

Another message reportedly said ... "Imma stab someone and lick the blood off the blade."

But damaging evidence about Austin was also kept from jurors .... KERA reports he had previously spray-painted racist and antisemitic graffiti ... including the words, "kill all black people."

Prosecutors testified both sides had agreed to keep inflammatory character evidence about the teens out of the trial.

Less than two hours after Karmelo's girlfriend raised concerns about him, he fatally stabbed Austin during a confrontation at a Frisco high school track meet.

As TMZ previously reported ... Karmelo admitted the stabbing but claimed self-defense. A jury convicted him of murder in June ... sentencing him to 35 years in prison.

His new attorneys are now trying to get that conviction tossed and secure a new trial.