Has there been any official or credible reports as to when the last COVID test was negative?
There have been multiple responses by people with official positions in the administration. On October 9, the Associated Press published an article entitled Why won’t White House say when Trump last tested negative? So, no, we don't know when President Trump last tested negative. Specifically, the AP cited these responses:
“Yeah, I’m not going to give you a detailed readout with timestamps every time the president’s tested,” White House press secretary Kayleigh McEnany told reporters last weekend.
“I can’t reveal that at this time,” echoed Alyssa Farah, the White House director of strategic communications. “Doctors would like to keep it private.”
“I don’t want to go backwards,” said Dr. Sean Conley, the president’s physician.
We do have an idea about President Trump's testing frequency (prior to his testing positive, at least), from the same AP article:
Senior White House staff and those who are in direct contact with the president are tested for the virus daily. The White House originally gave the impression that Trump, too, was tested every day, with McEnany claiming in July that Trump was “the most tested man in America” and tested “multiple times a day.” But Trump contradicted her, saying, “I do probably on average a test every two days, three days.”
With that uncertainty in testing frequency, it's hard to pinpoint the day of the last negative test, even though we know when the first positive test took place: October 1st after returning from Bedminster (again from the AP article):
Here’s what is known: On Wednesday, Sept. 30, during a trip to Minnesota for a fundraiser and rally, one of the president’s closest aides, Hope Hicks, began feeling ill. She isolated herself aboard Air Force Once during the trip home, but the White House appears to have taken no further action.
The next morning, Hicks was again tested for the virus. This time, the results came back positive, just as the president was about to leave for a fundraiser at his golf club in Bedminster, New Jersey. A frantic effort was made to swap out staff who had been in close contact with Hicks, including McEnany. But Trump, who had also been with Hicks, nonetheless boarded Marine One, along with other White House staff still in the dark about Hicks’ diagnosis.
After returning home from Bedminster, Trump was administered a rapid test, followed by a more accurate confirming test, which takes several hours to process. Both came back positive.
“Safe to say, his first positive test was upon return or at least after Bedminster,” McEnany said.