If it's cornered it will bite. First it will flee (can't), then it will show the rings you can see in the picture to warn predators. If it does that and you still touch it, or worse, pick it up, it will bite.

I'm speaking for a human here but I'm very bitey. From this sample size being 100% bitey we have to assume its the same for the blue-ringed octopus.

The biteyness increases when one gets moved out of the zone one can breathe in.

It looks friendly. I say go for it.