This can be done without keys and Muenchian Grouping:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet [ <!ENTITY nbsp "&#x00A0;"> ]>
<xsl:stylesheet 
    version="1.0" 
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
    xmlns:msxml="urn:schemas-microsoft-com:xslt"
    xmlns:umbraco.library="urn:umbraco.library" xmlns:Exslt.ExsltCommon="urn:Exslt.ExsltCommon" xmlns:Exslt.ExsltDatesAndTimes="urn:Exslt.ExsltDatesAndTimes" xmlns:Exslt.ExsltMath="urn:Exslt.ExsltMath" xmlns:Exslt.ExsltRegularExpressions="urn:Exslt.ExsltRegularExpressions" xmlns:Exslt.ExsltStrings="urn:Exslt.ExsltStrings" xmlns:Exslt.ExsltSets="urn:Exslt.ExsltSets" 
    exclude-result-prefixes="msxml umbraco.library Exslt.ExsltCommon Exslt.ExsltDatesAndTimes Exslt.ExsltMath Exslt.ExsltRegularExpressions Exslt.ExsltStrings Exslt.ExsltSets ">


<xsl:output method="xml" omit-xml-declaration="yes" indent="yes"/>

<xsl:param name="currentPage"/>
<xsl:variable name="numberOfLatestPosts" select="3" />
<xsl:variable name="blogPostSelection">
    <posts>
        <xsl:for-each select="$currentPage/ancestor::node[@nodeName='Home']//node[@nodeTypeAlias='BlogFolder']//node[@nodeTypeAlias='BlogPost' and position() &lt; $numberOfLatestPosts]">
            <xsl:sort select="./data[@alias='publishDate']" order="descending" />
            <xsl:copy-of select="." />
        </xsl:for-each>
    </posts>
</xsl:variable>
<xsl:variable name="blogPosts" select="msxml:node-set($blogPostSelection)" />

<xsl:template match="/">
    <table>
        <tr>
            <th>Date</th>
            <th>Header</th>
        </tr>
        <xsl:apply-templates select="$blogPosts//node" />
    </table>
</xsl:template>

<xsl:template match="node">
    <xsl:variable name="pos" select="position()" />
    <tr>
        <!-- Date -->
        <td>
            <xsl:if test="$pos = 1">
                <xsl:value-of select="./data[@alias='publishDate']" /><br/>
            </xsl:if>
            <xsl:if test="$pos &gt; 1">
                <xsl:variable name="prevDate" select="$blogPosts//node[number($pos) - 1]/data[@alias='publishDate']" />
                <xsl:if test="$prevDate != ./data[@alias='publishDate']">
                    <xsl:value-of select="./data[@alias='publishDate']" />
                </xsl:if><br/>
            </xsl:if>
        </td>
        <!-- Header -->
        <td>
            <xsl:value-of select="./data[@alias='header']" />
        </td>
    </tr>
</xsl:template>

</xsl:stylesheet>

This code creates a variable containing a list of blogPost nodes from the various blogs that is sorted by a date in a set property. (You can use @createDate instead if you like, but then you'll have to do a substring comparison on the datepart of the attribute in the last if statement)

This variable is converted into a node set with the msxml:node-set function, and this nodeset is then listed out.

Regards
Jesper Hauge