While it is true that black hats can easily download and study the source code of Linux, they are not the only ones who can do so. Any company who stands to gain from using Linux can hire white hats (security experts with ``good'' intentions) to audit Linux for security purposes. In fact, Linux is inspected for security issues by many organizations. Any found flaws are fixed in the next release for everyone to download.
There are also claims that because anyone can contribute to an open source project, black hats can ``contribute'' security holes. This is entirely untrue. Contributions to an open source project are managed by a core team. It is highly unlikely that malicious contributions will be incorporated into an open source project. Note that malicious programmers can also be hired as a part of the development team of a proprietary program.