well, boiling hot water did the trick!
The blade was covered with probably very old nitrocellulose lacquer or even shellac. A collector friend from Austria suggested that I tried that before the chemical way (which I wasn't too keen to undertake) and lo and behold, it worked. It came loose and I removed the deposit with a brush and now the blade is waiting to go on its journey by being treated with Warangan (it is a Bali Kris) from a local expert. Another step will be finding a nice sheath or sarong for it but that is another bridge which I will have to cross in the future.
Thanks for everyone's contribution