Skim milk or BSA would be fine for blocking. Have no experience of using IgG as blocking.

Michael, it probably depends on what you are going to do with your microbeads. Can you elaborate a little?

I am conjugating an antibody to beads for the detection of small quantities of a protein by an ELISA-type assay.

I read somewhere that IgG is used as a blocking agent when high specificity is required, but haven't been able find much else about it.

I would have though that IgG may cross-react with other IgG antibodies.

Supposedly, it's all about background signals if I haven't missed something. Gut feeling (experience from developing ELISAs and LF) tells me that if you add unspecific (but polyclonal) IgG there is some chance of  getting a nasty background. I'd rather use BSA or similar (as Xinjie suggested)) and possibly include an non-ionic detergent like Tween 20 for additional blocking.

I agree with Wolfgang, be careful for polyclonal responses to high amounts of blocking with BSA. We prefer BSA over skim milk powder, since it is purified. Try optimising the block incubation and concentration of BSA. Good luck!