I have always known that intrinsically safe terminals in a panel should be blue. Is it necessary to have blue terminals in junction boxes in the field as well?
I don't think it's mandatory to have blue terminals. The chances are, though, the whole junction box will be IS because you are not allowed to mix IS and non-IS wiring in the same wire duct (home run cable). You should identify the box as Intrinsically Safe.
The requirement is to clearly identify IS circuits and elements of circuits. This is commonly done by using blue as an identifying colour - but I am unaware of any mandatory requirements to use blue - it's more of a convention. In a JB it is probably more important to have a prominent label on the outside stating that "This Junction Box contains Intrinsically Safe Circuits." Unless the box contains a mixture of IS and non-IS circuits - not a good idea but not forbidden either, as long as there is adequate separation between the IS and non-IS stuff. Similarly in a panel it is more important to make sure that the trunking to be used for IS protected wiring is clearly defined.
ISA RP 12.6/NEC 504 calls for light blue colored terminals at the field JBs for IS circuits as well as blue colored outer sheather for all the IS signal cables.
