Generally blocking is always required before adding a primary antibody for immunofluorescence. The block is always commonly kept in the antibody diluent for the primary and secondary antibodies. Depending on your antibody it may be best to use normal serum from the same species that your secondaryantibody was raised in. I generally use 5% BSA, however. Here is a page describing best practices for blocking:

https://www.thermofisher.com/us/en/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/blocking-strategies-ihc.html

Thank you Jordan for the reply...I have a question.. when staining procedure is by keeping cells in suspension...then also blocking is required?

blocking is required even if your staining suspension cells.

thank you so much kunal..