Cryogen comes with Markdown support by default and you can only use one or the other.
If you want to use AsciiDoc instead of Markdown, open the project.clj in your created blog and change the line in :dependencies that says cryogen-flexmark to cryogen-asciidoc.
Instead of looking for files ending in .md in the content/md directory, the compiler will now look in the content/asc directory for files ending in .adoc. (For backwards compatibility, it actually also looks for .ad, .asciidoc, and .asc. But only .adoc is the official file extension, and newer features such as xref only work with that.)
If you want to configure Cryogen's use of AsciiDoc or create AsciiDoc extensions, see the repository for cryogen-asciidoc.