Like PeterH says, I don't see that frequency has anything to do with it, what you want to take is
a ratio of analog values. Unless some devices use modulated frequency to drive the Leds, and
bandpass filters to help reject background light, like in TV remotes.
However, the Sparkfun device you mentioned has nothing whatsoever to do with oximetery. The part
where it says "The TSL235R light-to-frequency converter outputs a square wave (50% duty cycle) with
frequency directly proportional to light intensity (irradiance)", the frequency is simply how they are
transferring the intensity information, so it can be read with a digital device rather than an A/D
converter.