Can you explain your use case a bit more of why you would have 250 databases to pull into a single view?
Yes, Tableau does have data blending, and yes it does have cross-database joins within the data dialogue, however it's not intended to be a full integration suite. Generally, if you have this many databases (perhaps one for each field office?), then you should have some kind of integration method upstream of Tableau. You can look at the Tableau Partners page for a list of suppliers within this space.