Fair Foul blocked from view

[KHanneman]

I work both baseball and slow pitch softball. This happened in a softball game, but could happen in baseball....
Ball hit down right field line, I knew it was going to be close call, so I was running down first base line to get a clear view of where the ball lands. I'm about 8 feet from first base, I stop to see the ball land- and the first baseman at the last minute steps into my line of view. I get completely blocked, but can tell it was a coin toss. The batter keeps running, the right fielder never lets up on the play, so I let it play out. The first baseman was the only one that makes any motion that its a foul ball.
First baseman looks at me wondering why I didn't call it foul and I tell him that he blocked my view and I didn't see it land.
So, what would you have called? And how do I prevent this from happening again?
By the way, the play didn't affect the outcome of the game, it was a blowout.