Blood on outside of eyeball- it's usually OK.

Blood inside eyeball (and you WON'T see that) which is suddenly a lot of black floaters in your vision, part of your vision gone, large cloudy bits in your vision, bright flashes, basically anything that causes your vision to change- BAD. That's your retina haemoragging or your retina detaching.

I did a series of short, not very high level plane flights over a couple of days and I ended up with a lovely red spot on my eyeball too!