How to add alkaline to a drink?
I had a look at the actual scientific paper describing the use of this substance as a pH indicator and a few other blogs, this is the overall advice:

1 - Please make sure that the FINAL pH of the solution is NOT basic-ish. This will not sit well in your customer's stomachs if they consume a full dose of a cocktail (120-150mL) with a final pH of more than 7.5. As I mentioned in the comments, there is a reason why you don't ingest toothpaste. Basic substances also leave a very unpleasant mouthfeel - it is a mix between a very tannin-ish feel and corrosive irritation.

2 - Do NOT add Alka-setzer. One person tried to do that and it ended-up being a gray-white shade, very unappealing for food and/or drinks

3 - There is not enough documentation on the REVERSIBILITY of this process (not all pH indicators are reversible), so you're going to need to experiment with it

4 - Aim for a final pH level of  5-6, this is where the blue color is at its optimal level. I suggest using CaOH (pickling lime / limewater) because that's easy to handle and nearly flavorless. Note that this is used in a lot of clarification processes, so you need to choose mixers that are low in protein so you don't get nasty precipitation when you add your limewater.

Reference:

The Butterfly Pea Flower as a pH Indicator