I rule if there is a light source and something to block that source there is a shadow i.e the sun shining behind a house you get a shadow in the pitch black dungeon you don't.
A fine houserule, just as long as you are aware it is a houserule.
While the name of the ability has the word "shadow" in it, that has no bearing on its function by RAW. The rules simply mention areas of dim light or darkness, so 'porting around in pitch black dungeons is completely within the rule. (The "that you can see", however, might still allow you to rule a lightsource is needed. I mean, you can rule whatever you want, but "allow" in the sense that consensus allows you to make a ruling without entering houserule territory. Myself, I prefer to interprete the "that you can see" clause to mean "to which you have line of sight" so I don't have to answer deep philosophical questions on whether you can actually see darkness at all
)
Generally any area of dim-light or darkness is a shadow of some sort.
As your personal opinion or ruling, this is fine. Do be aware, however, that some DMs will rule that given a sufficiently thick cloud cover, there are no shadows regardless of whether that light is considered bright or dim.
I stick to the "dim-light or darkness" rule because it makes the ability generally useless in a lot of situations. If there is not a specific area of dim-light the monk can't use it. The monk in my party is a wood-elf so she has dark vision to 60' but a human shadow monk would not be able to shadow step to an area of complete darkness because of that pesky "that you can see" clause.
Again, please don't assume your personal rulings are universal. You stick to that rule because
you rule it generally useless in a lot of situations. It is you who rule that the "that you can see" clause is "pesky".
After all, can you see darkness? How far "into" darkness can you see? Would a human monk be restricted to just 'porting into the first 5 ft space of darkness (along the outer edge of the light source's dim light)?
Not saying there is a right or wrong way to answer these questions (so please, no need to actually provide any answers). Just pointing out that it's hard to judge others for using houserules in an edition which continually requires yourself to make rulings all the time, easily making you fall into what others might percieve as houserule territory yourself.
I myself prefer to distance myself from close ties to "reality". Making judgement calls about shadows and darkness only makes the game harder and slower to run. Besides, the ability is clearly magical anyway, so why obsess over "reality". Much better to just allow the player to point to the map and teleport to any area of dim light and darkness within range.
Or (as I've already said) for areas where lightning isn't clearly marked on the map, roll a die: 1-3 = area is brightly lit, 4-6 = area is shadowy (dimly lit or outright darkness).
