She was

"Oh... she was blonde, was she?"

The Doctor listened to Martha and Jack. It was good to hear Rose being talked about, as if.... as if she still existed. But of course she still existed, just locked away in another universe. Forever. Martha left, he met Donna again. He started to almost enjoy himself again. he was free to talk about Rose. Donna encouraged it.

They had a mad time together. Saved people, planets. Until that day. Donna's alternate universe. He asked a final urgent question. He couldn't believe her response.

"She was... blonde."