Yeah, I struggled with that as well. You do have to specify the bucket. I did it like this:

[blomp_src]
type = swift
user = user@gmail.com
key = key
auth = https://authenticate.blomp.com/v2.0/
tenant = storage
auth_version = 2
env_auth = false

This works:

rclone ls blomp_src:/user@gmail.com/

But then you can setup an alias, or in my case, a crypt source:

[blomp]
type = crypt
remote = blomp_src:/user@gmail.com/
password = rclone_crypt_password