One key witness in the Aniah Blanchard case says he initially kept his mouth shut instead of going to cops because his wife told him it was best to "stay out of it."

The horrible revelation was just made in an Alabama courtroom Wednesday, where officials are trying to prove Ibraheem Yazeed forcibly shoved 19-year-old Aniah Blanchard into her car on Oct. 23 -- the last night she was seen in public.

Blanchard -- who's still missing and at this point presumed dead -- is the stepdaughter of UFC fighter Walt Harris.

During Wednesday's hearing, officials say the witness saw Yazeed forcing Aniah into her car at the Chevron station and leaving the scene ... but he did NOT immediately go to police. Instead, he confided in his wife who told him it was best to not get involved. Unclear why she felt that way.

Eventually, the witness had a change of heart and went to authorities -- and broke down in tears for not coming forward sooner.

As for the case against Yazeed, prosecutors say they found blood from TWO people in Aniah's car and they want to get a DNA sample from Yazeed to see if he's a match.

During Wednesday's hearing, the judge granted the request -- essentially forcing Yazeed to comply. The judge also denied Yazeed's bond, meaning he'll remain behind bars.

As we previously reported, cops say they found a large amount of blood in Aniah's car that matches Aniah -- and the scene was "indicative of someone suffering a life-threatening injury."

The judge also ruled the case will move forward to a grand jury.

As we previously reported, officials are trying to pin the disappearance of Blanchard on 29-year-old Yazeed ... saying he was the one who took Aniah at a Chevron station in Auburn, Alabama last month.