It is however true that there are substances which both fit on a blotter, and are indole psychedelics - e.g. 5-MeO-AMT, or other (other than LSD, that is) ergolines, such as LSB, ALD-52 (though these are extremely rare and (likely) share the safety profile of LSD). The Ehrlich test is not capable of distinguishing between different indoles, so you may get a case of mistaken identity in this scenario.
Edit: This PDF: http://www.bvda.com/EN/download/np_instructions_2002.pdf says the test indicates the presence of ergot alkaloids, not tryptamines.
Both the blotter paper and the inks on it could have a taste. Also, the sensation of taste (or lack thereof) could be influenced psychosomatically, by conscious or subconscious expectations.
Anyway, by the time you taste it, it's too late.
It's certainly preferable to identify a substance before tasting it, but it's not too late - spitting the paper out and rinsing the mouth out is likely at least to reduce the absorbed dose.
A perhaps more pressing problem is the risk of anxiety due to an innocently-bitter piece of blotter - I imagine a borderline-bad trip could be made much worse by a persistent fear that it would last >24h.
Of course, real caution would mean not eating anything cooked up by criminals in an illicit lab...
The point about bitterness is absolutely correct though, and certainly any purported LSD blotter that tastes bitter should be suspect.
Interestingly, at least one non-psychedelic has been found distributed on blotter paper: alprazolam (See http://www.justice.gov/dea/pr/micrograms/2008/mg0508.pdf), for which I'd tentatively expect the bitterness rule also to work.