I dug out LOTR the other day and started play from where we left off some time ago. This time it was much more difficult and wondered if event cards could be played to effect the other player if it doesn't say "your hero" or similar. Could an event card like "Blade Mastery" be played on another player's hero or ally?

Sure can.

I dug out LOTR the other day and started play from where we left off some time ago. This time it was much more difficult and wondered if event cards could be played to effect the other player if it doesn't say "your hero" or similar. Could an event card like "Blade Mastery" be played on another player's hero or ally?

You can, but keep in mind certain cards require you to pay a cost. If a card has the structure "Do X to Y" only the player playing the card is eligible to pay the cost.

One example is We Are Not Idle: "Exhaust X Dwarves to...". Only the player playing the event is allowed to exhaust Dwarves for this effect even though it doesn't say "You exhaust X Dwarves to...."

Now, the secondary portion of We Are Not Idle says "...to add X resources to a hero's resource pool". You are allowed to add those resources to any hero's resource pool since it doesn't specify one of your heroes.