I have started the Final Preparations quest and have done Triss and Ciri quest. Then I started Through Time and Space. Now I have read that I must do Blindingly Obvious before Through Time and Space. Is that corerct? What will happend if I continue with Through Time and Space, can I do Blindingly Obvious after?

No, you can do any of the final preparations quests, in any order. What Through Time and Space opens is 'Battle Preparations', which locks (or fails) any incomplete Final Preparations, but not automatically. You just need to complete everything else from Final Preparations before you meet the Lodge for Battle Preparations -- you will return to Novigrad after TT&S. Though, because of the quest flow, it is prudent to leave Through Time and Space until last, since it's what opens the next chapter.