what's the deal w/ higherbloods w/ psionics. how does that happen even. what's the serkets' deal having vision eightfold AND mind control AND reading thoughts/emotions and it's technically only one power. (aranea can read emotions and mindfang can both read minds and emotions, while vriska only claims to be able to do both of these)
Some highbloods can get psionics just because that’s a thing that happens sometimes with highbloods!
All trolls have the proper biological infrastructure for psychic powers (a psychic organ in the brain, various ‘piping’ accompanying the nervous system, etc.); it’s just that different castes and caste groupings have different likelihoods of developing different forms of psychic expression. In the cast of cobalt trolls, sometimes this manifests itself in an ability to read and manipulate other trolls’ brains.
In the words of Aranea Serket;
So the power itself seems to be the ability to simply access another troll’s mind, and what exactly the user chooses to do with this access is entirely up to them. In some cases, it can be used to actually take control of the mind, and in other cases simply to observe the things happening there.
The user of the powers can choose to train themselves in any aspect of this power, and with more training and practice, this ability becomes easier and easier to use. For a troll who practices a lot in reading the emotions or thoughts of another troll, it may become natural to them over time to the point where they do it without intending to. In extreme cases, the same might happen to a troll who dabbles in mind control a lot.
Vision Eightfold seems to be a completely separate thing that enables one to see through certain mystical barriers (such as the exterior of a juju like the Magic Cue Ball). Its exact nature is unknown, but one can extrapolate certain likelihoods based on implications about similar mutations; namely Vision Twofold. The main application of Twofold was allowing its user (namely Sollux) to see not only into the future, but also into the real world - thus subverting the Blind Seer fate that befalls oracles who do not have the same mutation, like Terezi Pyrope (who only had vision Onefold, because her sight was destroyed).
So one can assume that Vision Eightfold allows the user to see into seven realms or facets of reality in addition to standard visual sight. Whether this must include the same facets that Twofold includes (i.e. possible futures) is unclear, but the fact that Aranea states she doesn’t have precognitive vision (in the page linked above) would suggest that this is not the case.
One very likely explanation is that Eightfold allows the user to see into the realms of probability (tying into seven additional realms, as a lucky number) - possibly meaning users of Eightfold are able to see into seven possible futures, and make calls on which will be the most likely outcome of a situation based on which outcome occurs in the most possible futures. This works well with Mindfang’s ability to see into the Magic Cue Ball; magic balls are objects that operate on a basis of probability, having a dice on the inside, so it is most likely Mindfang was actually able to see which result the Cue Ball’s dice was most likely going to land on, rather than actually being able to see what the dice landed on.
(Though of course, being Magic, the Cue Ball’s dice always has a 100% chance of landing on something, i.e the right answer. Which means for all intents and purposes, in the case of the Cue Ball, Mindfang could see the answer.)
Of course, it would be absolutely plausible for a troll to use their Vision Eightfold to predict what someone is thinking or feeling; and in this case the lines between the powers could become blurred in individuals like the Serkets. Likely the Serkets do actually use these powers in conjunction and they are intertwined rather like scent and taste - but another troll with only a nose would still be able to smell, and a troll with only a tongue would still be able to taste. So to speak.