Revit preview not displayed

[SeanP]

It is because the geometry you are creating is only “Dynamo” geometry and you have not translated that back into “Revit” geometry or families. The blue lines you see in Revit are the previews of nodes that create lines and points.

Revit Dynamo created elements are not usable

As @SeanP said, geometry created in Dynamo are not actually built in Revit until you specifically do so using nodes. Yes, there is a preview of the geometry in Revit as a blue/purplish line but that is just a preview. It can be disabled under View -> Background 3D Preview -> Available Previews -> Revit Background Preview. To make it into a model or detail line within Revit, use the node ModelCurve.ByCurve or DetailCurve.ByCurve respectively.