https://gitlab.synchro.net/main/sbbs/-/issues/1007#note_7872
The problem appears to be line 391 in frame.js...
```javascript
if(attr)
this.__properties__.data[py][px].attr = attr;
```
Since (dark) black on black has a value of zero, it doesn't actually get set.
This change was apparently made in 2011 with commit 72488196a51
--- SBBSecho 3.31-Linux
* Origin: Vertrauen - [vert/cvs/bbs].synchro.net (1:103/705)