fck_internal.ccs
.FCK__ShowBlocks p,
.FCK__ShowBlocks div,
.FCK__ShowBlocks pre,
.FCK__ShowBlocks address,
.FCK__ShowBlocks blockquote,
.FCK__ShowBlocks h1,
.FCK__ShowBlocks h2,
.FCK__ShowBlocks h3,
.FCK__ShowBlocks h4,
.FCK__ShowBlocks h5,
.FCK__ShowBlocks h6
{
background-repeat: no-repeat;
border: 1px dotted gray;
padding-top: 8px;
padding-left: 8px;
}oFCKeditor.Config['EditorAreaStyles']

Re: How to change color of ShowBlocks
.FCK__ShowBlocks p, .FCK__ShowBlocks div, .FCK__ShowBlocks pre, .FCK__ShowBlocks address, .FCK__ShowBlocks blockquote, .FCK__ShowBlocks h1, .FCK__ShowBlocks h2, .FCK__ShowBlocks h3, .FCK__ShowBlocks h4, .FCK__ShowBlocks h5, .FCK__ShowBlocks h6 { background-repeat: no-repeat; border: 1px dotted expression(256-parseInt(document.body.backgroundColor, 10)); padding-top: 8px; padding-left: 8px; }In this I'm just trying to get to the opposite spectrum of the colors for the border. I haven't used the parseInt very much so I'm not sure how it handles hex and I'm not sure of the color range but you get the idea.
I haven't tested this so the script is probably a little off. But this may get what you want.
Good Luck