You can use SugarCube's markup.
Blah blah ''bold'' blah blah.
Or html.
Blah blah <strong>bold</strong> blah blah.
/% or %/
Blah blah <b>bold</b> blah blah.
/% or %/
Blah blah <span style="font-weight:bold;">bold</span> blah blah.
Or custom style markup.
Blah blah @@font-weight:bold;bold@@ blah blah.
Or some combination of html or custom style markup and CSS.
Blah blah @@.bold;bold@@ blah blah.
/% or %/
Blah blah <span class="bold">bold</span> blah blah.
/* in stylesheet */
.bold { font-weight: bold; }
The first solution is probably the simplest, and is my recommendation.