It'll be a blanket with the contours of a double bass.
On a more serious note, when I raised this as a possible problem with Allianz over the phone, the guy at the other end said something along the lines that he had to say that for every instrument they insure and that he cannot say anything that was off-script. The implication I took from that and a few other things he said was that so long as you've taken reasonable steps to safeguard your bass then you will still be insured despite it being technically visible inside the car.
But then don't trust a random person on a basschat forum parroting third-hand information. Phone Allianz up yourself to confirm and you'll probably get a similar response to me.
