Was used in the medieval period, although red was the most popular. It is made easily enough by melting beeswax with verdigris (Copper acetate), of which I have rather a lot. Other recipes are available; an authoritative text is “Guide to British Medieval Seals”, P D A Harvey and Andrew Macguinness, which says on page 17,
“Seals in medieval Britain were always impressed in beeswax, to which in the later middle ages resin was added, and occaisionally they contain fine hairs, presumably to strengthen the wax.”
The resin, according to a late medieval recipe, was mixed with oil, presumably olive or linseed, then added to the beeswax.
Here you see the blue verdigris (visible at top right) turning green when in the wax, which is good because I was getting a bit worried that all my blue verdigris, the kind formed by leaving copper sealed in a jar with acetic acid wouldn’t turn green, which is formed when air is allowed into the vessel with the copper and acetic acid. A lovely deep green it is too.
Now all I need are some official documents and a seal to use on it.
(It is important to note too that medieval and post-medieval sealing wax wasn’t melted, but warmed in the hands or by candle until soft, and then used)