Are there any cold tolerant trees or shrubs that will flower all summer long...
Well, the short answer is 'no", there are no shrubs that will bloom all summer. Some perennial flowers come very close to being ever-blooming, but even these need to take a break occasionally.
If you want to have flowers all season your best bet is to plant a variety of shrubs that have different bloom times. This way you can have a successive series of blooms. There could be a variety of shapes, forms and colors that would attract a wide array of pollinating insects.
If you like the looks of certain flowers - like hydrangea for instance - you can plant a number of different kinds of them. Some bloom in the spring, others bloom late spring, others begin to set flowers in summer and their flowers persist into the fall. They can remain on the plant throughout the winter, and provide something interesting to look at amid the snow-banks.
Here is a link that describes the various hardy hydrangeas that do well in our area. Please make sure that you pick the ones that are hardy in zone 3b or 4. Zone 5 plants may be able to survive in southern Minnesota, provided they are well protected through the winter.
I hope this is helpful. Please contact AaE again if you h ave further questions.