Hello,
It is not a jacket, but is a bodice, the top of a 2 piece dress and would likely have had a matching skirt. The buttons may have been replaced over the years (not sure of course) as I would expect shank buttons for this garment (buttons with no through holes in front). Hard to date it exactly as it is laying down flat, but it appears to be late 1880s time period.
It certainly could have been worn by a lady while in mourning, but my guess is was not intended strictly for that purpose. People wore a lot of black clothing for all ocassions and purposes.
In the close up photo, it looks like silk faille.