Blue RTV

It's bluer.

Some automotive RTV's have "regular" (makes acetic acid during curing, bad for oxygen sensors) and "oxygen sensor safe" (no acetic acid so OK for oxygen sensors) varieties, and some make the no-acetic acid version blue. But I would never assume that blue automatically meant "no acetic acid".

Just looking in my Dow and GE catalogs, there are about 200 different varieties of RTV in common circulation, so "blue" vs "black" may not be enough to distinguish between all of them. Heck, I have a hard enough time seeing the 10 resistor color codes :-).

Tim.