An opponent takes control of a character which has Venomous Blade attached. Will be Venomous Blade discarded?

Venomous Blade:
After Venomous Blade comes out of Shadows,attach it to a character you control. Then,choose and kill an opponent's character with printed STR 2 or less. Response: After you lose a challenge,Venomous Blade goes into Shadows.

Yes it will be discarded. The key text is ' attach it to a character you control'

This question comes up every so often because "attach to a character you control" and other similar phrases on attachment cards tends to sound like a one-time limitation on the action of attaching the card, rather than a continuous requirement on what the card is allowed to be attached to. Despite the wonky wording, the FAQ does say that these are permanent requirements, not just requirements when the attachment first comes into play.

Is it required to have that verbiage for templating purposes in order to enable it to attach at all, since it is coming out of shadows rather than a vanilla attachment being played from hand that would enjoy the default attaching rules?

Yes, as the FAQ states, you attach it by its passive (or response, in the case of most), there is no 'regular' opportunity to attach it.

Thanks for your answers.

I was confused by "After Venomous Blade comes out of Shadows". Looks like passive.

Thanks for your answers.

I was confused by "After Venomous Blade comes out of Shadows". Looks like passive.

Well, it's a little odd in that it a passive effect that initiates when the card comes out of Shadows and attaches to another card. So it looks like a play restriction for resolving the passive, only. However, the FAQ tells us that such restrictions should be read as constant attachment restrictions.

What could get confusing, though, is that if it were worded as a target requirement (eg, "After Venomous Blade comes out of Shadows, choose a character you control and attach Venomous Blade to that character. Then..."), it would only matter when choosing the target, not become a constant attachment restriction.