If I found a blank money order, is that the same as theft?

You have the right to remain silent. Only idiots talk to investigators/cops. Remember, no matter what you say it will be twisted around and used against you later.

No that's theft

Wrong the banks keep track of who bought the money orders so the true owner could be found. You knew better and should have handed it over to the police.

You did more the possess a blank check. You cashed a negotiable instrument that did not belong to you. That is called theft.

You will not find the phrase "possession is 9/10 of the law" in any statute, legal cannon, or constitution. Whoever the money order was intended for, it wasn't yours to cash. As a matter of fact, you may even be in violation of the law as some states and towns have laws governing the steps someone must take when they find lost property. It would be the same as if you found an envelope full of cash and kept it. It's not yours, plain and simple. You have no legal right to claim it and keep it as your own.