I think the key 'missing chord' that so many have been searching for is the jazzy and
appropriately 'blue' Em7b5/Bb used in the second line of the chorus, the bridge
and then it's the penultimate chord of the song.

Em7b5/Bb   x12030 or x1203x

In the 'boasting of loving' section I think Costello strums F#7 (note NOT F#7 minor) throughout -
I've added alternate chords in brackets that follow the bass line and harmonise but I don't think 
Costello plays this.


[Intro stays on D]

             
 Now it's just

             
 you and me, m

       
y blue friend

 And you say t

hat it's you t

hat she's thin

king of
          
 And our affai

r must end

 But if it's y

ou that she's 

thinking of
             
 I think my br

              
oken heart mig

   
ht mend

             
Now it's my tu

           
rn to talk and

           
 your turn to 


think
            
Your turn to b

       
uy and my turn

   
 to drink
            
Your turn to c

#         
ry and my turn

    
 to sink
             
 Down in the B

    
lue Chair
            
 Down in the B

    
lue Chair

Now I've made up my mind
I've made my mistake
And I know that she cries for you
When she's barely awake
Well she's going to bend your mind
Well I hope it don't break

[Repeat chorus]

              
Down in the Bl

   
ue Chair
         
We can watch o

   
ur troubles ri

se
         
Like smoke int

      
o the air
          
And drift up t

  
o the ceiling
              
Down in the Bl

   
ue Chair
         
You can feel j

           
ust like a boy

/     
 or a man
         
The next minut

          
e you can find

      
 yourself knee

ling
              
Down in the Bl

   
ue Chair
          
They're boasti

          
ng of loving t

     
he daylights
            
right out of h

        
er in the smal

   
l hours
              
Down in the Bl

   
ue Chair
           
You say that y

             
our love lasts

    
 forever
            
when you know 

             
the night is j

          
ust hours     

     
uh ho ho ho ho

 ho

And still I want her right now
Not any minute, hour or day
And wherever she is tonight
I want her anyway
I suppose she never said to you,
You were just in the way

             
Now it's my tu

           
rn to talk and

           
 your turn to 


think
            
Your turn to b

       
uy and my turn

   
 to drink
            
Your turn to c

#         
ry and my turn

    
 to sink
             
 Down in the B

   
lue Chair
            
 Down in the B

    
lue Chair
             
 Down in the B

   
lue Chair
           
 Down in the B

        
lue Chair

            
 Down in the B

    
lue Chair
            
 Down in the B

    
lue Chair
            
 Down in the B

    
lue Chair

    
  Down in the 

blue
        
 Blue becomes 

you
     
 Down in the B

    
lue Chair