>I also read about it being used in image compression. If you split an image into R, G, and B channels, you can save the B at half resolution and the difference is nearly imperceptible to human eyes, whereas it is easily seen if you do that with R or G.

Never heard of chroma subsampling happening in RGB, generally that's converted to something like YUV which is a Y channel of luminance (about 58% derived from the green channel) and two UV chrominance channels. Both channels of chrominance are then subsampled horizontally by 50%, reducing 1/3 of your original data without even getting to the compression stage. Pretty much all compressed video you ever watch has the chrominance subsampled 50% vertically as well, meaning you've cut the original data size in half.

Point is, not just blue getting subsampled, and since that's what everyone is constantly watching it's pretty safe to say it turned out alright.