If you had the blazer altered, you should have been given both a receipt and a claim ticket. If I am understanding this correctly, they are stating that the item must have been picked up because you do not have the claim ticket and the jacket is no longer there.
Maybe I am wrong but if the manager is able she should trace what happened to it. I mean the blazer does have a stock number and she could ask someone (maybe her general manager) to locate the stock number and find out what happened to it instead of assuming that you picked it up and just do not remember doing so.

Please let us know the outcome of this. It will be interesting to see if someone in the upper management handles this issue differently.