Personally I think Black foam core is a little too rigid,blackout cloth
with velcro probably works best,...to each their own.

Covering a window with black material is a bad idea, unless it is always in
shade. Black material sealed behind glass will turn sunlight into enough
heat to melt and weaken plastics and cause outgassing in some materials.  I
used black material on a south window during a Minnesota winter and it still
got hot. All you need is anything that is opaque and reflective. Tape? Do
not use 'duct' tape or masking tape. It leaves a sticky residue. 3M makes a
tape specifically for window sealing. See their window winterizing kits. It
is made to be removed without damage.

Hope this helps.

In the temporary bathroom/darkrooms I've had over the years, I just used
a piece of 1/8 inch masonite a little smaller than the window opening
with foam weather stripping affixed around the edges, and wedged it
into the window opening.  A small handle or knob attached to the center
of the masonite helped getting it in and out of the opening.  Worked
really well, even in the brightest of daylight.  No leaks.

 What's wrong with cardboard? Cut it a little big and it'll hold itself in
place. Use a heavy drape over that and no light. Easy to remove. No damage.
 
 Nick