Ahh. The joys of Keynote's transitions... tbh, not it's strongest suit.
I think I understand your ask...
You have two slides, each slide:
- Contains one text block.
- Text block has a Build-in action set to Dissolve such that it builds your text block one paragraph at a time.
You have a transition between slide 1 and slide 2 to dissolve, but the problem is this dissolves Slide 1 to black, then the first paragraph in the second slide begins to appear, and it's this interstitial black you're trying to avoid.
The solution is simple, but not initially obvious.
On the second slide you need TWO text boxes.
The first text contains the first paragraph only, and has no build-in effect applied.
The second text box contains the additional paragraphs, and has the Build In -> Dissolve applied.
Now when you transition from Slide 1 -> Slide 2, the paragraphs on slide 1 simultaneously dissolve out while the fixed paragraph/text box on slide 2 appears.
Once the transition is complete, the additional paragraphs in the second text box dissolve in one-by-one.
By creating fixed items on the slide that have no build-in effect applied, Keynote uses this as the basis for the slide transition.
Does that help?
