You can't have list of items with unrelated types (note that in case of class F<T>{}
Foo<One>
and Foo<Other>
are not related to each other since there is no common parent).
Since IDataflowBlock
is only shared base class/interface for your types you have to do:
var listOfTransformBlocks = new List<IDataflowBlock> {...}
Unfortunately you'll lose compile-time ability to find types of your transformation and have to write some run-time code to find them back.