I've found that if I create a painting on the iPad using a blend mode that doesn't exist on the iPhone, for example the pencil tool in dissolve mode, I can export the AR file to Dropbox and from Dropbox open the file in AR on the iPhone and continue drawing in dissolve blend mode on the iPhone ...

This ability also extends to tools not resident on the iPhone, such as watercolor: if I create a painting with the watercolor tool on the iPad, then export to Dropbox and open in AR on the iPhone, I can continue using the watercolor tool on the iPhone, and in the iPhone tool set there will appear a label named "Watercolor" that points to an empty box ...

Tools and blend modes "borrowed" from the iPad are limited in scope; you can only use the tool on the same layer you've imported the image into, and if you switch to any other tool (except for the color palette) you cannot go back to the "borrowed" tool or blend mode ...