Blender VSE imports default to CROSS as blend mode (Set the default Blend Type)
Multiple strips cannot be edited simultaneously via the Strip Properties' "Edit Strip".
I was not able to find information on whether the VSE selection can be used from Python (e.g. a "selected" property or method), only
If the strips in question share e.g. a prefix, they can be modified like so via the Python console:
>>> for strip in D.scenes["Scene"].sequence_editor.sequences_all: ... if strip.name.startswith("myimage"): ... strip.blend_type="ALPHA_OVER"
Is there another way, or can the selected strips be accessed via the Python API?