AIUI, the point is that a sane dose of LSD doesn't taste bitter, so if one rejects any bitter blotter then one would be erring on the side of caution - possibly rejecting perfectly good LSD due to a bitter ink or paper, but avoiding the great majority non-LSD psychedelics.
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...
