I thinkk you want

<xsl:for-each-group select="*" group-adjacent="starts-with(local-name(),'b')">
<xsl:choose>
<xsl:when test="current-grouping-key()">
<b>
<xsl:copy-of select="current-group()"/>
</b>
<xsl:otherwise>
<xsl:copy-of select="current-group()"/>
</xsl:otherwise>
</xsl:choose>
</xsl:for-each-group>

David