What will happen if I block off the crankcase breather pipe on a Triumph Legend?

Dont be tempted to plug it as it will pressurise the crankcase and cause all sort's of problems. The breather does what its name says, allows the crankcase to breath.
You could end up with oil blowing past the piston rings or blowing oil seals.

 

usually a sign of excessive blow by on pistons although shouldn't happen on such low mileage? i would have a compression test done

 

Are you sure you dont have to much oil in, as that could cause it to blow it out of the breather.

 

You could try running the breather pipe into a plastic bottle of sorts and securing it to the frame but dont block the end of the bottle, (the pipes got to breath) and take it for a run and check how much oil its putting out.
My Tiger 1050 used to breath a bit of oil ( about a teaspoon) into the air box every couple of month's but didnt seem to bother the bike or the performance. I had the bike from new and part ex it 5 year later, I just kept an eye on the level.

 

Is that on the side stand or when the bike is upright?

 

Having same problem T/Bird sport.

 

you could put one of these on (or similar if wrong size)
12mm MINI OIL AIR INTAKE CRANKCASE VENT COVER BREATHER FILTER SILVER | eBay