In beagle bone black there are 3 i2c's, i2c-0,1,2
And out of this you're using i2c-1, So according to the linux images of beaglebone black the i2c-0 may appear as i2c-1 and i2c-1 appear as i2c-2 and so on , So I suggest you that instead of using i2cdetect -r 1 try
i2cdetect -r 2
so it might show the slave address you're looking for .... 0x68
And don't forget to add the DS1307 driver for the RTC in the kernel you're building if you're using a custom kernel