Also, can you tell if there is any gum residue on the back?

Hello Delia, the color and shading look closest to the example in
Cherrystone Auctions, Mar 2009, Sale 200903, Lot 209 and there is no gum residue on the back but only a small rectangular transparent "tape like" area as shown in my example "watermark"

Hello Delia, the color and shading look closest to the example in
Cherrystone Auctions, Mar 2009, Sale 200903, Lot 209 and there is no gum residue on the back but only a small rectangular transparent "tape like" area as shown in my example "watermark"

this small area could be residual gum?

Delia, I have been in possession of these books of antique world stamps for more than 20 years and have only just begun to look at them seriously! You may imagine that I am not and expert? lol.... so, I believe the small rectangle gum on the back of the Washington 5c Blue is a hinge! I have another stamp coming your way, a Purple 3c Washington!

Yes, I'll be doing that one later today. :)
Thank you for confirming the color - its quite helpful.
I'm not sure how many corner stamps are in existence - there isn't a census on that - so it's hard to know how much rarity is a factor; that yours isn't in mint condition keeps the value modest. Otherwise, you're all set for this one.