If you look at Electra/Bitcoin transaction history:

https://screenshots.firefox.com/RbL8EhpOjal2i0CB/coinsmarkets.com

There are 3 transactions made today on exactly the same amount of coins. This is probably a test made by the developers.

It will not replace it, instead blockchain can improve supermarkets in general, with the possibility to track product origin easily for instance. With all the possibilities that are generated with this technology I'm pretty sure we will see some changes and improvements in a couple years.