I want a tattoo on my forearm...but everyone says it will be a job killer. But I wear long sleeves at my job anyhow (business shirt) and sometimes I even wear a suit into work..sooooooo....

The only way someone from work would see the tattoo is if they saw me outside of work.

And im ina specific field so if I go for a new job, my attire will remain the same.

Also, there a lot of professional jobs where you wear long sleeves and tattoos on the lower arms would be hidden. Unless you work in labour or a more blue collar job where t-shirts are permitted, but it doesn't matter if you have a tattoo for most jobs of that sort anyway.

So how would a forearm tattoo really affect me in job prospects? Business pool parties???

it wont for the reasons you have stated!

Plus they are very commonplace these days

enjoy the ink burdda

It can, and it probably will.
A lot of jobs are coming down on those who get visible tattoos on forearms as well as other places that aren't usually covered up.
I had to sign a form saying that I do not have any visible tattoos on my body while working, otherwise I wouldn't be welcome to work there.
Meaning that if I got a tattoo done after the fact of me working there, they would probably fire me.

I personally wouldn't do it because a lot of people and employers are highly discouraged by it.
You would never want to jeopardize a possible good job opportunity for a bit of ink that you could get on a less visible spot of your body.

Edit: It would also help if you were to ask your employer what his views on it are. Then you would find out for sure before possibly sticking your foot in a trap.
Good luck though!