No, it's not an argumentum ad ignorantiam.
The argumentum ad ignorantiam assume that since something has not been proven false, it is therefore true. Conversely, such an argument may assume that since something has not been proven true, it is therefore false. It assumes that all propositions must either be known to be true or known to be false. Lack of proof is not proof.
There are a few types of reasoning which resemble the fallacy of Appeal to Ignorance, and need to be distinguished from it:
The burden of proof is usually on a person making a new or improbable claim, and the presumption may be that such a claim is false. For instance, suppose that someone claims that the president was taken by flying saucer to another planet, but when challenged can supply no evidence of this unusual trip. It would not be an Appeal to Ignorance for you to reason that, since there is no evidence that the president visited another planet, therefore he probably didn't do so. Another example: Invisible Pink Unicorn's incarnation requires both that Invisible Pink Unicorn become human and that Invisible Pink Unicorn remain wholly other. There is no evidence of Invisible Pink Unicorn's incarnation, therefore he probably didn't do so.
When extensive investigation has been undertaken, it is often reasonable to infer that something is false based upon a lack of positive evidence for it:
If there really were a large and unusual type of animal in Loch Ness, then we would have undeniable evidence of it by now. We don't have undeniable evidence of a large, unfamiliar animal in Loch Ness. Therefore, there is no such animal.
Another example:
If there really were Invisible Pink Unicorn, then humanity would have undeniable evidence of it by now. We don't have undeniable evidence of Invisible Pink Unicorn.
Therefore, there is no Invisible Pink Unicorn.
