I have always used mushroom manure as mulch. Works great, and adds organic material to the beds for the next season. This season I am going to cut costs and use semi-composted leaves from the local dump... problem is that I might be adopting noxious seeds from other folk's yards. I'll deal with that as it comes.

Here's a link to a FAQ Sheet, with two questions about mulch.. There is a long list of mulching material growers have used.Dahlia Growing Season FAQ

Keep in mind that many large growers simply hoe or rototill until the dahlia plants are large enough to shade out the weeds. Mulch helps, but is not necessary.