TL;DR: The fix is for Sorcerer Bloodlines to modify their Granted Spells with the Shadow Trait.
So now that the APG is out I went to roll up a sorcerer with the shadow bloodline.https://2e.aonprd.com/Bloodlines.aspx?ID=14
I check out it's level one bloodline spell and its Dim The Light, whenever you cast a darkness or shadow spell you can use a reaction to mess with the light in an area and immediately hide. That's pretty cool, and I start making my character with an eye towards utilizing it. It's only later that I discover that darkness and shadow refer to a specific trait of spells, and as a level one shadow sorcerer I don't have the ability to learn any spells with those traits. What gives? The initial bloodline spells are universally designed to provide a minor boost that any sorcerer of that bloodline can utilize. Well maybe things get better once I get 2nd level spells? Hardly, there's exactly one 2nd level spell with either descriptor and it's Darkness. How exactly does it benefit me to momentarily fuss with the lights to give a +1 to stealth a benefit when I'm already engulfing an area in blackness? It seems useless in the majority of situations you would use it in! At 5th level you finally get Chilling Darkness which seems to be the earliest opportunity to actually use your initial spell in some cool ways, but it's only after months of playing in real world time.
Now context would suggest to me that this is unintended. If it's suppose to be the case that the earliest feasible use isn't until 5th level the it should be the Advanced Bloodline Spell. Further, flavor wise it doesn't fit. Chill Touch, the bloodline's cantrip, is described as "...your hand radiates a pale darkness" and Grim Tendrils at 1st level starts with "tendrils of darkness curl out from your fingertips.' Furthermore, the shadow blood magic specifically saids that these spells have the additional effect of making shadows grow deeper.
Obviously I'm going to try to argue for a DM Fiat in the meantime, but it seems that officially there should be some sort of errata fix for this. I can think of 3 potential options:
1) The most narrow fix would be to change the requirements one Dim The Light to include spells granted by the bloodline.
2) You could change the Blood Magic description to add 'any spells that don't already have it gain the shadow trait.'
3) More broadly maybe it's Chill Touch and Grim Tendrils who should have the darkness trait given that their descriptions explicitly call out that you're manifesting it with your attack.