Could be Jackmanii, first or early flowers can look funny on my clems it happens occasionally, especially when we have late frosts.

Yup..I agree with BoS..looks like Clematis "Jackmanii"

Here is a link that might be useful: Jackmanii

The alternate flower form is very common. I tend to see those types of alternate forms more in the summer once it heats up although I do have some flowers on Durandii right now that look very different than the other blooms. Its been a weird spring so far however with periods of intense heat alternating with periods of cooler weather. That type of temp. vacillation will cause weird flower forms on some clematis. As to its id, could be Jackmanii but there are way to many purpley blue clematis out there to say with any certainty.