Can I still end up playing with a blocked player if a party leader invites the person I blocked

Yes. Blocking only make it so they can't communicate with you, but you can still end up in games with them.

That's clearly not the question I asked is it, if a person has been blocked they shouldnt be able to invite you to anything yet they still could so why is this able to happen? I received constant invites to games and parties of someone that wasnt on my
friends list and was blocked after the first few I didnt respond for over 10 minutes of the party invite message popping up on my screen

I find when I block someone I haven't seem them again. But if I am in a party and the leader hasn't blocked them then I will some times get matched up people I have blocked. I have a few times been a game when I am in a group go to block a bad player for
one reason or another and go oh he/she is already blocked.

Also on some games with small player pools it may be that you see a blocked player more often.

If you don’t want to receive communications from a specific player on Xbox One, you can block that player. Do this by selecting the player’s gamertag from within the game and blocking the player. If the person is no longer playing the game, you can search
and block the person through the Profile app.

The block only applies to communications such as messaging. The blocked player will not be able to send you messages, unless you first send a message to that player. However, a blocked player can still join games with you.

Source: Block a gamertag

the can still send party invites if you block them , I had over 200 notifications