If referring to physical transparency:
* Opaque: This means something does not allow light to pass through it.
* Cloudy: This describes something that is not clear or translucent.
* Solid: This describes something that is not hollow or porous.
If referring to metaphorical transparency:
* Opaque: This means something is difficult to understand or unclear.
* Hidden: This means something is concealed or kept secret.
* Secret: This describes something that is not shared or revealed.
* Dishonest: This describes something that is not truthful or open.
* Evasive: This describes someone who avoids giving direct answers.
The best antonym will depend on the specific situation and the intended meaning.