Can I put any antibiotic cream on poison ivy blisters that are healingi'm on day 4 of 20mg prednisone? How can I keep from getting an infection?

Ok to use. No problem except for the potential of developing a contact allergic reaction to the antibiotic in the cream with neomycin being the most likely issue. Just keep the area clean by washing with soap and water would often do the job.
Keep clean, covered. Keep the wounds clean and covered. Don't scratch them or anywhere else. These are fairly low risk for infection if you do this with what you are already doing. Topical antibiotics unlikely to help you.