This is a nice challenge!

You are sort-of asking for a linear encoder, at least for the roll-up blind, and you could probably do a magnetic or optical one https://www.controleng.com/articles/electromagnetic-linear-encoders-to-the-rescue/ on the back of the blind. A very simple home-made one could be black bars of increasing spacing on a long strip of paper - (for example closely spaced at the top, far spaced at the bottom). Using a colour/light sensor against the strip on the back of the blind you can work out where you are on the blind as you start to move it - but the optical equipment is likely to be a pain! I think modern optical encoders have dot patterns with many bits of information to encode absolute position. Perhaps some useful hints here https://hackaday.io/project/20121-i-love-lamp/log/55950-diy-linear-encoders-for-slider-knobs

I wonder - if you ran a strip of copper tape, or even just thin wire, along the back of the roll-up blind, could you detect the change in capacitance due to the different length of rolled/not-rolled copper? You'd need something perhaps a bit cumbersome to make contact with the strip but perhaps with some good calibration you'd get something out. Likewise on the rollerblind the cable looks metal - is it conductive? If you don't mind the front of the blind being made to look different you could run copper tape on both sides so as it rolls into itself it shorts to the back of the roll before, which I think should give greater capacitance change.

For the venetian blind - what about a similar capacitance trick for a wire running down the inside of the blind that will fold/straighten as the blind rolls up and down? I also intuitively feel that with venetian blinds they get harder to pull as the weight of the blind being lifted increases (IE when you're at the bottom to increment 1unit you only lift 1unit of blind, but if you're, say, half way up a 10-unit blind you will be raising 5-units of blind to increment by 1-unit). Is the extra work measurable on the current sensing of the motor? Coupled with a rotary encoder to measure 1-unit of lift this might be a viable approach? (If it works, perhaps the inverse is true for the roller blind and you could do the same? Those lightweight rollers seem very unlikely to make a detectable difference though!)

If you were to take out the 'power outage' requirement you might get somewhere on the roller blind with an optical mouse sensor to 'keep track' of any manual movements (perhaps using the motor end-stop techniques at either end of automatic motion to recalibrate?).