In that plugin's file, find this:

			if (!b2evoCheckOpenTags(i)) {
				var URL = prompt( '<?php echo T_('URL') ?>:', defaultValue);
				if (URL)
				{
					b2evoButtons[i].tagStart = '<a href="' + URL + '">';
					b2evoInsertTag(myField, i);
				}
			}

Now make it be this:

			if (!b2evoCheckOpenTags(i)) {
				var URL = prompt( '<?php echo T_('URL') ?>:', defaultValue);
				if (URL)
				{
					b2evoButtons[i].tagStart = '<a href="' + URL + '" target="_blank">';
					b2evoInsertTag(myField, i);
				}
			}

Untested so you should backup your file before doing this.

thanks, that worked perfectly. I knew one of you experts would have the answer in seconds!