Hi Joyce,
I think the way you need to approach this is by deciding what the real problem is.
Forgive me, I know I am going to sound sanctimonious in writing, but believe me I am being sincere in message!
Is the problem that the blackbirds may be taking the baby birds, or that you don't like the fact that the blackbirds may be taking the baby birds?
I know that might come across as a bit horrid, but I think it is an important distinction, because it might help determine what you what to do.
I understand it is very easy to think of your garden or yard visitors as "your" birds - I do it myself - but they are no more yours than you are "their" humans.
The nesting birds have taking advantage of birdhouse you have put up, and that, along with many other factors, will have helped them choose their nesting site.
It's not very nice to see (or be aware of) animals taking other animals as food, but it happens, and it is perfectly natural. It may be, that all things considered, the nesting birds have decided that even on the chance that the blackbirds might take their young, that your birdhouse is the best chance they have of successfully raising a brood.
That said, if the nest hole is small enough, then the young are probably as safe inside your birdhouse as they would be anywhere. Just because you don't see the parents coming and going, doesn't mean they are not. Also if the blackbirds are in the habit of taking nestlings, then it will happen as much in the wild, and not because of your birdhouse.
That in turn said, a well stocked bird table may go some way to reduce the blackbird's requirement to take smaller nestlings, and may be something positive that you can do.
Now, I am by no means an expert, but other than the above, I think you can do nothing else this breeding season.
If you were to move the birdhouse, or put some kind of guard over it, then the disturbance may well be just as disasterous as you fear the blackbird's interference may be.
In you position, I would wait a year and see what develops, then is necessary make any changes in time for next year.
Anyway, whatever you decide, if you can live with the thought that the blackbirds may be taking the nestlings, then you may well have the breeding birds and the blackbirds both as garden visitors.
If you really do want to discourage the blackbirds, then you might lose them altogether, as they go off in search of another place to feed.
But whatever you decide, I hope you ultimately get what you want, and I hope I have gone some way towards helping you achieve that, without sounding too nasty! :eek!:
With luck, someone else will be along soon with some more advice.
By the way though... you say "House Sparrows". Do you mean our European House Sparrows, because if you do, you seem to be unusual amongst our American members in welcoming this little "invader"!!!
All the best.