Can you post a picture- that would tell us immediately if it was a tuber or a bulb. Some tubers can be really round & look almost like a bulb especially if it's a pot tuber.
If you can't post a picture then go to the Colorado Dahlia Society site & look at the tubers on a few of the pages where the digging instructions are & see if any look like what you purchased.
Since it's really late to be getting dahlias in the northern hemisphere I'd wonder if you weren't sold something else.

Here is a link that might be useful: page 4 digging dahlias