Appeal to ignorance is an argument in favour or against a proposition due to lack of evidence against or in favour of the proposition. If it be the case that there is positive evidence for the proposition or conclusion, then the reasons for accepting it are somewhat different.
But in the case when there is no evidence, the state of the conclusion is null and void or is what it was prior to the initiation of the proposal. Appeal to ignorance has its basis on the presumed state of the conclusion i.e. the state prior to the proposition. If something is assumed to be true and no evidence exists to prove otherwise, then the conclusion remains true else, it is false.
Example: If I have no reason to think that Miss Katrina has a crush on me, that is not evidence that she does not have a crush on me. Hence, the latter cannot be ruled out in entirety.