Separate 4 cups of black locust flowers from their stems, removing any stray leaves that may have been included in the mix, but leaving the flower sepals (green base)
Pour 4 cups boiling water over the top of the black locust flowers and then wait 15 minutes before straining. It'll be clear or slightly green/yellow-ish at this point, but don't worry, the color will pop when you add the lemon juice.
Pour the strained tea in to a saucepan and add the lemon juice. The lemon juice adjusts the color and flavor of the flowers, making them taste amazing, but also adding acidity to help preserve the jelly.
Bring the mixture to a boil and add the powdered pectin, stirring to dissolve. Allow the mixture to boil for 1 minute before adding sugar.
(Note: Do not add the sugar quite yet, allow the pectin to boil 1 minute first.)
Add the sugar, stirring to dissolve (See notes on quantity). Bring the mixture back to a full boil for 1 minute before ladling it into jelly jars leaving 1/4 inch headspace.
If canning, process in a water bath canner for 10 minutes (or 15 minutes above 6,000 feet in elevation). Otherwise, allow the jars to cool completely on the counter before storing in the refrigerator (for up to a month) or the freezer for up to 6 months.