The idea of the BCC field is indeed to keep recipients undisclosed from each other. But it should not be understood as a reliable security feature and I wouldn't trust it in a life-and-death situation.
Servers often add additional headers to emails during processing. These headers might, as a side effect, inadvertently reveal BCC recipients to everyone else. For example, this has been a problem with various email encryption implementations. Some research on that can be found in the paper Correcting Privacy Violations in Blind-Carbon-Copy (BCC) Encrypted Email. From the abstract:
We show that many widely deployed email encryption systems reveal the
identities of Blind- Carbon-Copy (BCC) recipients. For example,
encrypted email sent using Microsoft Outlook completely exposes the
identity of every BCC recipient. Additionally, several
implementations of PGP expose the full name and email address of BCC
I could imagine that similar problems exist with anti-spam filters or other forms of email processing that your mail server might employ.
Also, there have been flawed implementations in the past. For example, Microsoft Outlook Express Information Disclosure Vulnerability
Microsoft Outlook Express 6.0 SP1 and prior contain a vulnerability that could cause private information to be disclosed to e-mail recipients. The application may arbitrarily allow a recipient to view the addresses listed in the BCC: field.
As an alternative I'd suggest you to send the mail to each BCC recipient individually. If there are many of them, you can use a bulk email plugin for your mail client that will send the mails one by one.
You can find a related answer here.