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Black Ball : To vote against allowing someone to be a member of an organization or group, you are blackballing him or her.

Blank Cheque: To give a blank cheque to someone is to allow them to use much money as they like for something, or do as they please.

Blank Slate: Something that hasn't been developed as yet or described in any detail.

Bleeding edge: A technology or process that is at the forefront or beyong current practices, but because it is unproven, it is often dangerous to use, hence 'bleeding'.

Bleeding heart: Someone who is very sympathetic towards other people.

Bleesing in disguise : A misfortune or bad luck that eventually has positive results or consequences.

Blind acceptance : To accept something without the slightest of doubt.

Blind leading the blind : When those in charge of something don't know anything more than the people they are in charge of.

Blind-sided: To be taken completely by surprise by some event with a negative impact.

Blink of an eye : SOmething that happens so fast that it is almost impossible to notice it.

Blood and thunder : An emotional speect or performance.

Blood from a turnip: Just as you can't get blood from a turnip, you can't get something from someone when they don't have it.

Blood is thicker than water: It means that family relationships are stronger than others.

Blood out of a stone : When something is like geting blood out of a stone, it is very difficult.

Blood, sweat and tears: SOmething that is very difficult and requires a lot of effort and sacrifice.

Blow a fuse : To become uncontrollably angry.

Blow by blow : A blow-by-blow discription give every detail in sequence.

Blow hot and cold : When someone blows hot and cold, their attitude and opinions keep changing very quickly.

Blow and down: People say 'blow me donw' when they hear something surprising, shocking or unexpected.

Blow off steam: To express anger or frustration.

Blow smoke: To exaggerate or say things that are not true, usually done to make themselves look better.

Blow the cobwebs away : To make sweeping changes to something to bring fresh views and ideas in.

Dear Friends those above idioms are very informative for you

Thanks
Ibanker
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If you have written these explanations yourself, that's great, but if you have borrowed them from another source, please provide the source so that no one thinks we're trying to steal their ideas. Thanks.
Comments  
That's a great list! I'm sure it will be very helpful to many!
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 BarbaraPA's reply was promoted to an answer.
I have borrowed them from Daily English Dawn.