When does the Blind reward count for interest?
The next interest payment uses gold before the reward is added. The reward can raise the following payment.
See your next interest step, what a purchase costs, and the cash after a known Blind reward.

Start with the gold shown in your Run. Add a price only when you are looking at a purchase.
Enter the gold shown in your Run. The amount you can spend before losing next interest appears immediately.
Interest is paid from gold held before the Blind reward. The planner shows the next step and how a purchase or reroll changes it.
Keeping gold helps later shops only if you can clear the next Blind. If a visible offer turns a losing hand into a winning one, compare that gain with the lost interest in the Run planner.
The Blind reward can fund the next shop and the following interest payment. Unused-hand cash, money Jokers, Tags and later purchases can change the final balance.
The next interest payment uses gold before the reward is added. The reward can raise the following payment.
No. Spend when a purchase is needed to clear the Blind or has more value for the Run than the interest you lose.