-always dark==>no eyes, and no bioluminescence as this was developed to attract creatures with eyes. However like deep sea shrimp they might have 'light sensor' on their backs, which can do little more than tell the creatures if there's a light source nearby. (Although their environment is pitch-black to human eyes, the hot vents emit an infrared glow.)
A source of nutrition-chemosynthesis or living off chemosynthesising organisms cf At their deep-sea homestead, the shrimp feed on gardens of bacteria they cultivate on their own bodies, a strategy also used by yeti crabs recently discovered at hydrothermal vents in the Antarctic.Then predators could live off these in turn.
-a way of moving-if strong currents exist a good description above of how they would need some (of the many possible) ways to hang onto the vent (claws, hooks, suckers etc)
-a way of sensing-most likely using vibration sense? or sound? (though I've no idea what happens to sound at huge depths and pressures). Likely is simple blind crawling around and feeling out for prey using touch organs. Also likely is simple slow-metabolism creatures lying in wait for someone to bump into them then ambush and snag and eat the unfortunate creature.
-also possible is upside-down creatures-as recently discovered in arctic-here underside of ice sheet-the 'roof' of their world is (as they swim 'upside down') now their 'ground'. Note 'upside down' is only our perspective, they are optimally orientated to their environment.
-ignore terms such as very cold and very high pressure-these terms mean nothing to a creature that has evolved in such an environment-for them this is perfectly normal.
-Equally when people say very cold=slow metabolism this is true of earth creatures, it may not be true of alien creatures. Earth has such 'weird' life-cycles just here that we can't begin to imagine how weird it will be elsewhere. Look at flabby whalefish and anglerfish for example.
-when people say only enough oxygen for small amount of creatures to live a) this is just an (educated) guess and b) life will thus live off something else, that seems to be the pattern here on Earth, complexity emerges out of whatever is there, under the right circumstances (which Europa seems to have).
-likewise I don't see any reason why they couldn't float/swim etc in this giant ocean, maybe simple organism with incredibly slow metabolism or even on the cusp of dead/alive (think Resurrection plant).
-creatures might also live off chemicals not just off the vent but those same chemicals released into the ocean eg not hunter/hunted but chemosynthesising jelly fish (who in turn might be prey for another creature/s)
-if large sea creatures existed, 'swimmingly' freely they would probably do so in a manner similar to our sea creatures-from simple drifting to guided activities using all the various methods of aquatic locomotion (see wikipedia) that have evolved here on Earth.
-if larger sea cretures existed as hunters they would very likely use a guided sensing method of hunting eg smell, vibration sense or even echolocation. Use of long (maybe very, very long) 'tendrils' that could act as a aquatic spider web also possible (why not?!)
-maybe no communicative color or markings-no eyes-no color needed, color on Earth has signalling function but there any color would be entirely random. Note how creatures around deep-sea vents (from what I know) are often color-free (white it seems) and have no markings.
-Other ideas, edited from Wikiepdia-'deep sea fish'...
...are sedentary, adapted to outputting minimum energy in a habitat with very little food or available energy.... Their bodies are elongated with weak, watery muscles and skeletal structures. Since so much of the fish is water, they are not compressed by the great pressures at these depths. They often have extensible, hinged jaws with recurved teeth. They are slimy, without scales. The central nervous system is confined to the lateral line and olfactory systems, the eyes are small and may not function, and gills, kidneys and hearts, and swimbladders are small or missing.
The swimbladders of deep sea fish are either absent or scarcely operational.
The most important sensory systems are usually the inner ear, which responds to sound, and the lateral line, which responds to changes in water pressure. The olfactory system can also be important for males who find females by smell.
Because food is so scarce, bathypelagic predators are not selective in their feeding habits, but grab whatever comes close enough.
It is not easy finding a mate in this zone....(some) are hermaphrodites, which doubles their chances of producing both eggs and sperm when an encounter occurs.
Many forms other than fish live in the bathypelagic zone, such as squid, large whales, octopuses, sponges, brachiopods, sea stars, and echinoids, but this zone is difficult for fish to live in.