I was a bit confused about your use of the term 25I because I have always understood the number to mean the number of fields or frames. On Wikipedia they have decided this is incorrect - news to me. Anyway, I think that this issue is also what has caused you confusion. I suspect that your camera produces 50I = 50 interlaced fields per second and according to Wikipedia Blue Ray Disc supports 25 interlaced frames per second - in other words what I term 50I. Therefore, your camera already produces a suitable format for BD as you would expect.
If you encode your video as 50I your BD player can only show it at 25P if it has built in I2P.
By the sound of it; you may have to use the lower field first option; but I don't know what kind of interpolation you refer to.