Certified GKH Gardening Expert
Did this happen after adding the ironite, or was this an attempt to correct the issue? This can be a pH, issue, but more commonly, there are infections that only affect the flowers.
Can you include photos? This will help me to see the issue. I can, then, give you the best advice about what to do.
In the meantime, this article will give you information about the care of these: https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-care-growing-and-planting-lilac-bush-plants.htm