Not only did I ask her out and we dated, she became the love of my life for 30 years. We first met in college and she was actually engaged to be married then. We hung out together and had mutual friends but over time the "light bulb came on" for me (and I think for her too). I didn't feel any strange transitions. We just fit together in so many ways. We were "just friends" for about 6 years before we dated and then tied the knot. I firmly believe that the way to a good marriage is to be good friends first. You figure a lot of things out without the stress.
I buried her two years ago and still terribly miss my best friend. We went through the gamut of the wedding vows in our time together. I sure wouldn't trade it for anything and wish we could have grown old together. Alas, God had a different plan. My best advice is to trust your instincts when it comes to your feelings about your friend. You have nothing to lose. You will even learn if it was a strong friendship or not if the dating doesn't work out.