User Manual LG 55LJ550M Full HD 1080p Smart LED TV

The C1 series sold in the US has a black back cover. However,
some other markets will have white so it's possible that any
unboxing videos you've seen were done in other markets,
where a white version will be sold.

I'm planning on using this as a computer monitor. Is burn in
still a problem or has it been fixed?

'Can be wall mounted' should mean that the tv comes with
an attachable stand but you could wall mount it instead if
you get yourself a wall mount, while 'wall mount' suggests
that the tv does not come with any stand. This tv does
come with the wide solid stand shown.

Don"t think it's fixed should be alright if you don't leave it on the
same screen for a long time.