Just tried i2c on the blue pill. i2c-init gives a proper "ok."-prompt, but "i2c." shows a lot of adresses of devices that do not exist. The driver is commented as "not working yet". May be this is the cause?
Correct: the hardware I2C driver for F103 is not working (JNZ/L052 is ok).
But you can use the bit-banged version (it doesn't support clock stretching).