I noticed that Ricky's fiance who pereished with him in the plane crash was named Helen Blair and David's first wife was named June Blair. Were the two related in any way?

I don't think so. There's seems to be nothing to be found about Helen Blair except she was "Rick Nelson's girlfriend". One of those bios (Joel Selvin's, I think) just mentioned that he met her at a show and after that, they were inseparable and she moved in with him. No info other than that.