I actually wasn't able to find the error message -- "invalid send amount" -- through internet searches.

But I think Blockchain.info engineers must be reading this thread.  Right before my eyes, the error message changed to the following:

-----------------------------
Insufficient Funds

This transaction cannot be sent because there are insufficent funds available. This may be due to additional miners fees or unconfirmed transactions.

The transaction can either be cancelled. Or the amount to send can be adjusted to match the maximum amount available to send.
Required: 0.xx312 BTC (Available: 0.xx262 BTC)
-----------------------------

I guess it didn't work because I was emptying my wallet without paying a transaction fee.  And I guess I have to do a little arithmetic to figure out what the transaction fee is.

It would be much more convenient for users of your wallet if you simply listed the transaction fee on the SEND MONEY screen along with a simple note explaining the transaction fee mandate.

EDIT:  That was weird.  After posting this, I went back, and the transaction worked without any fee.  I guess blockchain was updating their site.