Dick Powell and Joan Blondell didn't marry until September 19, 1936; what ended up being the final Powell/Keeler film, "Colleen", was released earlier that year. From January 4, 1933 to September 4, 1936 Blondell was married to her first husband, cinematographer George Barnes. The Powell/Blondell marriage was in trouble early on and they were both unfaithful. I agree that it's unlikely that Powell and Ruby Keeler were involved when they worked together mostly due to the jealous and possessive nature of Keeler's husband, Al Jolson (he apparently even hired private detectives to keep tabs on her). Jolson insisted that she leave Warner Brothers with him in 1937 and her career never fully recovered. Powell was apparently unaware of Jolson's massive ego and jealousy until they worked together on "Wonder Bar" (1934); Powell and the rest of the cast ended up despising Jolson for his behavior and upstaging antics.
I recently discovered that Powell and Keeler may have been involved following her separation from Jolson in 1939. It was noted in the press at the time that she moved into a house that was close to Powell's bachelor pad and it stirred up a "minor scandal". No doubt they would have tried to keep it secret for obvious reasons. I don't think Keeler would have had a fling under normal circumstances but given that Jolson had treated her so badly and they were headed for divorce, maybe she decided to enjoy herself. She and Powell had known each other for several years so it wasn't as if she was having an affair with a random stranger. (Keeler married her second husband, John Homer Lowe, in October 1941) There is, of course, no way to verify this all these years later, but I think it's safe to assume that there are a lot of stories that we don't know about. You'll notice that both books written about Ruby Keeler don't go into depth about her personal life and her relationship with others, as she was a very private person. Given that Powell was married at the time he likely wouldn't have talked about it either. He did mention in a 1937 interview that he missed working with Keeler because she was an empathetic actress and she made it easier for him to react to her because she reacted so well to him and he knew that their chemistry could not be duplicated. In the introduction to the pilot episode of his last television series, "The Dick Powell Show", he specifically mentioned Ruby - and only Ruby - when he talked about how his early movies were often seen on late night television.
The information about the possible affair between Powell and Keeler was noted by Powell's third wife, June Allyson, in her 1983 autobiography. She stated that when she was dating Powell his father loaned her a scrapbook about his early career. His "romantic triumphs" as she put it were also featured - Powell was quite the ladies' man in the 1930s and he did have a tendency to become romantically involved with his female co-stars. Mary Brian, Ginger Rogers, Rosemary Lane, Marion Davies, and of course Joan Blondell. I was very surprised to read that there may have been more than a professional relationship between Powell and Keeler. I can't say whether they were ever in love with each other but there may have well been an attraction and they seemed fond of one another.
Here are a couple of photos. The first one is candid of Powell, Keeler and Joan Blondell taken in 1935 which is interesting in retrospect, as is the next one of Powell, Keeler and Al Jolson (who is the only one looking at the camera and looks very possessive of his lovely young wife in the presence of the boyishly handsome Powell).