Seemed quite straightforward - the black haired girl was mistaken thinking her friend had got a boyfriend - she WAS easy to read in that she was acting different and all happy - but that was because blondie had fallen for HER.
The twist is it seemed the black haired girl was ALSO easy to read - her friend had noticed how sad she was when she thought she had a boyfriend, and how happy and relieved she was to find out it wasn't true - and how happy she is when asked to hang out etc..
In other words the blond girl realised the black haired girl was in love with her - it was probably what made her fall for her too. It ends with blondie debating if she should keep teasing her cute oblivious easy to read friend for a while longer.