Excellent! Thank you for the quick response and fix :)

For me only the first screenshot is black.

Mine were fine, but after last update they are all black or completely random good/black. In-game screenshots (not caught by steam) work fine

Hello, what game are you testing with?

Portal, Portal 2, HL2, HL

Is this still the case with the latest beta build that just released? If so, what graphics card and driver are you running?

So far so good, every attempt to take screenshots with the latest beta worked. Just FYI, since you asked:

Video Card:
Driver: NVIDIA Corporation GeForce GTX 750 Ti/PCIe/SSE2

Driver Version:  4.5.0 NVIDIA 346.22
OpenGL Version: 4.5
Desktop Color Depth: 24 bits per pixel
Monitor Refresh Rate: 60 Hz
VendorID:  0x10de
DeviceID:  0x1380
Number of Monitors:  1
Number of Video Cards Not Detected
Primary Display Resolution:  1920 x 1080
Desktop Resolution: 1920 x 1080
Primary Display Size: 20.51" x 11.54"  (23.50" diag)
                                        52.1cm x 29.3cm  (59.7cm diag)
Primary Bus: PCI Express 16x
Primary VRAM: 2048 MB
Supported MSAA Modes:  2x 4x 8x 16x 

Memory:
RAM: 6012 Mb

Miscellaneous:
UI Language: English
LANG: C
Microphone: Not set
Total Hard Disk Space Available: 225286 Mb
Largest Free Hard Disk Block: 54885 Mb

Thanks! We're refactoring the screenshot code a bit and are trying to iron out the last few quirks. Let us know if there's a game where it still doesn't work.

I spoke too soon. Black screenshots in HL2 after restarting (i.e. quitting and loading again).
Loaded Half Life - screenshots were ok - and HL2 back again - also back to normal.

Further investigation: in Half Life 2 the black screenshots start happening when the game has been running for quite a while or loads new map