Soil acidifiers are usually considered to be soil amendments rather than fertilizers but are often combined with fertilizers. And, some fertilizers have definite acidifying properties.

To obtain 'blue' flowers on a hydrangea, it is the aluminum levels in the soil and their ready availability to the plant that determines 'blueness', not soil acidity nor any specific type of fertilizer. However, soils which are not very acidic tend to bind or tie up the aluminum so that it is not easily accessible. Products known as "hydrangea bluing" or aluminum sulfate, combine aluminum with an acidifier (sulfur) to seal the deal.

It takes time for these elements to react with the plants, so even if the AS or acidifying amendments were added now it is doubtful they would have any effect on any additional flowers you might see this season.

Thanks, Garden Gal.