<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I had 5 pieces of cherry and about the same of maple. </div></BLOCKQUOTE>
I think you're right.
Ancho and chipotle will darken a bit when cooked, as will allspice; clove and pepper are already dark, but I'm not seeing this as an issue.
Splenda does not caramelize at all, unlike sugar. It goes from stable to burnt, period, but this happens only at much higher temps. (I could not reach the lab deparrtment at McNeil Nutritionals, Splenda's manufacturer, but the person I spoke to confirmed this though she could not give me an actual number. No one has ever asked before! She checked with someone else who thought 450 was its burn point. That, or perhaps slightly higher, makes sense to me.)
In the right conditions--a fairly full smoke stream, usually early on in the cook, coupled with cool(ish) temps (either smoke temps, meat surface temps, or both), plus the addition of moisture (the Worce and the normal moisture developed by the salt) can lead to what is called 'smudging'--black streaky deposits on the meat's surface. This continues as long as moisture is present if the rest of the conditionsare right. (This is why one forms a pellicle on belly and salmon, e.g., before low-temp smoking.)
I'd lean toward your suspicion. With the potential for the necessary conditions being right, plus the addition of a substantial amount of smoke, it seems quite possible that smudging might occur.
Try the same cook with substantially less wood and see what happens.