I don't like most of trick endings either.But there are a few ones that I find quite interesting, and this one is among them. The point and similarity of them is that: We didn't realize it's a trick until the killer confessed to his guilt, or made some fatal mistake. That kind of trick I really love! Other gems in that caterlogy are:

A Friend in Deed (Not particularly fond of this ep, but the final trick is amazing)
A Deadly State of Mind
Negative Reaction

These tricks really ring the bell to me.

I thought the laundry bluing was not good because a dentist is likely to have qualifications in stuff like physics and especially chemistry, so he'd either know it was a bluff or not admit guilt knowing it probably would be a bluff.