Hi Filip,
Blend is the ‘original’ blend command, and it is fairly simple in that you don’t get much control over the results. There is no SDK support for creating these types of blends.
BlendCrv is the newer command and provides the user with a lot more control over the resulting blend curve. There is SDK support for this in the form of RhinoSdkBlend::CurveBlend(), as you know.
Can you post a screen shot and/or some geometry that will give an indication of what you are trying to produce?