I've been told by my supplier that the diamond blades can only cut one way, is it true?

Your supplier either has no idea how those segments are made or knows very well that this is a nonsense, but hasn't got the know-how to achieve dual direction cutting or both.

Just ask him to repair a blade for you, to braze a new segment on and watch him how he pulls a carefully marked segment out of a box so that he places it in the right direction. Does he? :-)

Of course not! The segments are not made in any such way. He will pull them out from a box, where they float freely and even though you will find some arrow markings on the segments you will find there is no way of knowing which way they should be attached. Because there is no need to know.

The picture below is a photograph of a segment

The arrows you see are at both sides of the segment

Which way do they go? Either way, the orientation does not matter!

The truth of the matter is that after a prolonged period of cutting the diamond grains and the bonding matrix between them wear and shape themselves into a micro texture suited for a particular direction of cutting.

All you need is to reverse the direction and after a very short while they will rearrange themselves into the new direction.