IANAD - most of the really nasty stuff gets transmitted via blood and mucous membranes. Mono and COVID are probably the worst of it (via saliva). That being said, I still don’t trust birthday cakes and generally abstain (for multiple reasons). Unless I’m altered.

Also have to consider that these nasties die after some time living outside of the body.

First step is not to spit on the cake when you blow. Of course there will be some tiny, tiny amount that gets on the cake, but it shouldn’t be a lot.

You’d have a much bigger risk being on the other side of the cake and breathing the air from the person blowing. There are some things that can be transmitted by ingesting am infected person’s saliva, but many more things (including COVID) are spread by inhaling droplets, and don’t survive being ingested. Also, not many things are going to live long on cake frosting.

There’s some risk, but not a lot.

Never ever kiss your lover!