The 'real' meaning of blend level transparency integers

[David Glasgow]

Hello folks,

I am writing a psychological assessment which involves visual signal  
detection.  Basically, a 'blob' slowly appears at a random point  
within one of three invisible rectangles.  I have just about cracked  
the random generation of a loc within a given object.

What I am having trouble with is understanding exactly what the blend  
level integers represent.  I am not image savvy, and don't know if  
these are, or can be related to, any recognised and objective  
transparency units.  Any recommended reading on this sort of thing?

Two specific issues are

1/  A blendlevel set at 50 looks less than 50% transparent to me, and  
an incrementally decreasing transparency doesn't look linear to me.   
Is that to do with the technology or human visual perception?

2/  How confident can I be that the visual effect of incrementally  
decreasing transparency will be equivalent across machines and OSs?   
I understand this is comparing Win 2000 and later with OS X.  Is  
there anything I need to watch out for?

Thanks

David Glasgow