If you can, have the "computer" that you use for such things not matter if it gets hacked. If your blue ray player has no writable storage or network access and you power it off after every use, there is no danger
I don't think there's a single BD player out there that doesn't allow for either software updates or updates to the BD codes that allow/disallow you to decode disks.
One I have requires a USB key to be present to cache validity information for disks you have already watched - without it, it still works, but requires contacting the mothership through Internet whenever re-inserting any disks newer than the latest firmware update.
BD disks these days even come with extras like links to youtube videos, that play on the BD player. That's an attack vector right there. Do they all use https and check the validity of the cert to avoid MITM attacks, using only name servers with signed entries? I highly doubt it.
If I wanted to hack it, I feel fairly confident that I could do so. I'd start by hooking up to the (convenient) JTAG interface, and learn as much as i could that way, before starting to probe from the outside, i.e. through discs, USB or TCP/IP. But it would be low on my list if things I own that I want to hack. My car is more interesting.