I am using this derivative of Robert's eMMC -> img file script
https://elinux.org/BeagleBone_Black_Extracting_eMMC_contentsand it runs instantly and creates a 1K image file. I suspect something has changed since that Wiki entry was created.
I have tested this on both Element 14 and Beagleboard versions of BBB.
Here is my uEnv.txt file:
bootpart=0:1
bootdir=
fdtaddr=0x81FF0000
optargs=quiet capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN
uenvcmd=load mmc 0 ${loadaddr} uImage;run loadfdt;setenv bootargs console=${console} ${optargs};bootm ${loadaddr} - ${fdtaddr}
and I have
am335x-boneblack.dtb
in root and /dtbs
Thanks!