I found a Civil War Pension Card on both ancestry and fold3. The one on ancestry shows a William W Longnecker, Co D 52nd Illinois Infantry with widow Sarah Longnecker.
In 1891 William became an invalid; in 1895, Apr 6, there is an application no. 611720 and a certificate no 436815. Therefore, William Longnecker must have died before April 1895. Because Sarah Blackmore is not in the direct line I am researching, I don't think I'll order the pension record, but there may be other information there for all of you who are researching Sarah and/or William.