It's because of the package is not listed in the repository. You have to add the repository which contains the package. First open distfeeds.conf file:

vi /etc/opkg/distfeeds.conf

There are some url is commented. I'm not sure which one contains the package you need, so just uncomment all the url and save the file.

Then run opkg update to update the list.

Then you try to install block-mount again.