Can you bless somebody without their consent?

You can but depending on who they are they might take exception to it. A courteous bless you in conversation is unlikely to cause offence but if you inform someone that you are undertaking actual blessings on them then that certainly might rise hackles. It could be seen as negating the free will of the individual.

I'd avoid the interminable New Agey bless this and bless that and reserve the term for genuine blessings that you have either been asked to perform or that you have consent to undertake.

Incidentally the same goes for items. I'd be hopping mad if a witch shop cast blessings on ingredients I was going to use for magic. Well meaning as I am sure it is, it is interfering in what I want to do and it would annoy me greatly. 

The best advice I can give you is if in doubt, ask.

Image http://www.sxc.hu/photo/1438545 (lance1)

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