Danny Rose put the U's head in the 33rd minute with a free-kick from 25 yards and, after Tom Soares had equalised for the Shakers after 70 minutes, Johnny Mullins restored Oxford's advantage by turning in David Hunt's low free-kick 10 minutes from time.
Cameron was sent off when he received a second yellow card for protesting too much.
He had given away the free-kick which led to Mullins' winner, collecting a yellow card for the initial foul before being booked again for arguing when the hosts turned it into a goal.
Blackwell said: "The sending-off was absolutely pathetic. It was a nothing foul. As a manager you want the decisions to be equal. It can't be that as the away team decisions always go against you.
"But he'll be done two weeks' wages for getting sent off, in line with our policy."
On the game, the Bury boss added: "We came back from their goal in the first half, showed terrific character but just didn't finish off the approach play. The second half was one-way traffic.
"It disappoints me that we got nothing from the game because we were comfortable, even in the first half before Oxford scored. You've got to take something from games like that.
"We should be sitting here with six points from the first two games instead of none. We switched off for the free-kick at the second goal, which is an absolute crime."