Poor choice. Unless there's a compelling reason to stick with this, I'd strongly suggest to not use 19th century technology for this critical part.
Yeah, I get that...
The question is: how much light exactly is 100% transmission? Of course it's never 100%, but I get what you mean - a clear tape/film is defined by you as 100% and any coloration would result in a transmission of less than 100% in whatever color channel you measure.
OK, so you're working on blue now, but you have an incandescent lamp, so a broad spectrum light source (with very poor performance in blue, but let's put that aside for now). How do you 'single out' the blue light? A filter in front of the lamp, or the sensor?
You mentioned that you don't want to use an RGB color sensor. Could you explain why? I'm asking because there's about a 99.8% chance that this will make your life much easier. You're most likely reinventing the wheel, and doing it poorly if you compare your efforts with what e.g. an AS73211 has to offer.