Transparent Materials

In order to make the material transparent in Blender navigate to material properties panel and go to the Transparency section. Transparency is controled by the slider titled Alpha. The startup value is always 1.000, which represents an opaque material. Moving the slider to the left decreases that value making the material more transparent down to full transparency equal to 0.000.

The above screenshot shows the material named Glass with transparency set to 0.689. This value will be translated to 68 in the intermediate E file. Only two first digits after the decimal point are taken into account.

The default rendering engine of Blender (Blender Render) cannot display transparency in the 3D View, but it's possible to change it. First make sure you can see the right toolbox panel. If you can't see it, activate it by pressing N key. Scroll it down to the Display section and change the Shading to GLSL. Next change the 3D viewport's shading to Textured (see below):

How to debug?

Blender Console displays all materials with the detected transparency. See the screenshot below:

Example model

Below you can see a simplified model of aquarium. That primitive mesh has only two materials (Glass and Material) but the glass part is double-sided and transparent. The screenshot below shows the output of internal Blender renderer (don't pay attention to scene arrangement, light etc... it's been rendered only for preview purposes):

The same model can be exported by means of the Dark Exporter and placed in the Thief mission. Below you can see a screenshot of how that model looks in the Game Mode:

The faces representing the glass are set as double-sided in Blender. The material's Alpha parameter set up to 0.733 was translated by Dark Exporter to the value of 73. Below you can see the same model with textures applied and one additional abstract model placed inside the aquarium. The blue ball is also transparent (TRANSP 56). It was a testing model only: