It's easier to just modify the user control. Find \CMSModules\Blogs\Controls\BlogCommentEdit.ascx and the following is the control for the URL. Set them invisible.
<tr>
<td colspan="2">
<div class="BlogCommentUrl <%=LiveSiteCss%>">
<asp:Label ID="lblUrl" runat="server" AssociatedControlID="txtUrl" EnableViewState="false" />
</div>
<asp:Panel ID="pnlUrl" runat="server" DefaultButton="btnOk">
<cms:CMSTextBox ID="txtUrl" runat="server" CssClass="TextBoxField" MaxLength="450"
EnableViewState="false" />
</asp:Panel>
</td>
</tr>