The background blur property is still in beta and not fully supported by all browsers. For more details see: https://caniuse.com/#feat=css-backdrop-filter
Artifacts and Slowness
On browsers that do support background blur, certain usages might reduce the performance of the page significantly, or might show rendering artifacts. This is especially true when involved in animations or when working in the Framer canvas.
On new projects the background blur feature is disabled by default. It can be enabled via the menu under "View", "Show Background Blur". If the canvas in Framer appears to be slow when the feature is enabled, you may need to disable background blur.
Note: Disabling background blur will only make it disappear in the canvas, the feature will still show in preview, the viewing device, and in Prototype View.
Background Blur + Blending Modes
In general, Framer does not support combining background blur with a blending mode. If any layer in the hierarchy uses a blending mode, adding a background blur will create artifacts.