Hard to say - burnout fabrics have been around a long time, though I think they had a sort of revival especially late 90s-early 00s-ish... Prym is a German brand that has been around forever, they produce just about every kind of haberdashery item, mostly for home sewers, so these were probably buttons that you could cover with fabric yourself. Typical Prym product and unfortunately no help with dating. But it points towards the blouse being home-made, which would work with the fact that it has no labels.