I built something that solves this problem. It uses an open source program installed on the client to encrypt and store data in a way that doesn't allow the website to access the plaintext data. https://redact.ws
Nice! I worked on something that seems similar to this (https://redact.ws). Unfortunately there are a lot of challenges with adoption that seem difficult to overcome. One of the big challenges is that most people do not understand the nuances of privacy and encryption, and they aren't willing to jump through hoops to protect their data.