Support blurb class on divs

doc/src/boostbook.css

@@ -317,6 +317,7 @@ Blurbs
     div.important,
     div.caution,
     div.warning,
+    div.blurb,
     p.blurb
     {
         font-size: 9pt; /* A little bit smaller than the main text */
@@ -326,6 +327,7 @@ Blurbs
         padding: 0.5pc 0.5pc 0.5pc 0.5pc;
     }
 
+    div.blurb img,
     p.blurb img
     {
         padding: 1pt;
@@ -549,6 +551,7 @@ Colors
         div.important,
         div.caution,
         div.warning,
+        div.blurb,
         p.blurb
         {
             border: 1px solid #DCDCDC;