The rules for reading a Bloodline entry lists this:
Blood Magic: Whenever you cast a bloodline spell using a Focus Point or a sorcerous gift spell using a spell slot, you choose one blood magic effect you know to benefit from. You begin play with the benefit listed in your bloodline and can gain others through sorcerer feats.
Crossblooded Evolution is the feat mentioned that will cause you to know an additional blood magic effect. You could then choose that new bloodline effect for the blood magic effect when you cast a bloodline (Focus) spell or any of your Sorcerous Gift spells.
The initial Crossblooded Evolution feat doesn't give you any additional focus spells or Sorcerous Gift spells, so you are still only eligible for blood magic with the ones that you have from your initial bloodline (even if you do know and have available some of the spells that would be Sorcerous Gift spells from your second bloodline). I think this is what you are asking about, and I do agree with your assessment in your second paragraph.
You can get additional Sorcerous Gift spells with Greater Crossblooded Evolution. You could use any of your known blood magic effects with those new Sorcerous Gift spells in addition to the ones you have from your 1st level bloodline choice.
The initial Crossblooded Evolution feat doesn't give you any additional focus spells or Sorcerous Gift spells, so you are still only eligible for blood magic with the ones that you have from your initial bloodline (even if you do know and have available some of the spells that would be Sorcerous Gift spells from your second bloodline).
As a concrete example:
A Sorcerer with Elemental(fire) Bloodline would have Sorcerous Gift spells of Breathe Fire, Resist Energy, Fireball, and Unfettered Movement at level 8. Any of those would qualify for blood magic.
This Sorcerer might also know the spell Fear because it is available on the Primal spell list and can be chosen as one of their Rank 1 spells. Casting Fear wouldn't trigger blood magic effects.
If that Sorcerer takes Crossblooded Evolution at level 8 and chooses Diabolic bloodline, they would gain the Tongue of Flame effect that they could apply any time they cast their Elemental Bloodline Sorcereous Gift spells. They wouldn't be able to use it when they cast Fear because it isn't a Sorcerous Gift spell yet. They still only know and can cast the spell because they chose it as one of their freely chosen spells to put in Repertoire.
At level 18 the Sorcerer can take Greater Crossblooded Evolution for their Diabolic Bloodline and add Fear as one of their Sorcerous Gift spells. They would probably want to retrain the Repertoire slot that they have been using to cast Fear with for the previous 17 levels of gameplay. And they would be able to choose either the Tongue of Flame effect or the Elemental Fury effect when they cast Fear.
As a complete side note:
You add these spells to your spell repertoire, heightened to the highest rank of spells you can cast or to the highest rank they can be heightened to that is lower than the highest rank of spells you can cast.
That's the same thing. You can always heighten a spell to any Rank higher than its minimum. You just might not get any additional benefit for doing so.
You can heighten Fear to Rank 5. There is even some niche use reasons for doing so involving Counteract checks.
But by the troll ruling of Strict RAW, when you take Greater Crossblooded Evolution at level 18 and choose Fear as your bloodline Sorcerous Gift spell to add, it would be added to repertoire at Rank 9 because that is the highest Rank of spell you can cast and is also a Rank that it can be heightened to. And unless you choose it as your 9th Rank Signature Spell, then you can't use lower Rank spell slots to cast it with.
Here's a question: is anyone actually using bloodline abilities meaningfully? It seems like the effects are too minor and short lived given how limited your spell selection to trigger them is.