in a blog post

It should be on , I think

I prefer "in" here.

It's contained/ included within the post.

IN a blog post, ON a blog.

Yes, it's perfectly natural, WB.

Thank you JTT and McTag and others~

How about this?:

-On/In that TV programme on magic, the most enticing thing is the voice over.

Which preposition should I choose?

Awkward, isn't it?

The programme is on tonight. (It's going to be broadcast on TV: it's listed in the schedules)
The most interesting thing in the programme is the voice-over.

also

Nelson Mandela's funeral was on TV this morning. Desmond Tutu was on, giving a speech. In the service, there was lots of singing from the choir.

I think a general guide would be, if the meaning is "contained within", then use "in". "On" is used in phrases like on the blackboard, on the wall, on the TV, on the radio. (That is, not contained within the radio, but broadcast by means of the radio, therefore on the radio).

Thank you, Mctag~

Although to go slightly off-topic, to convey that particular thought we would probably say,

"The most interesting thing about the programme was the voice-over."

Thank you~

-There was also a TV program in which they were interviewed for their research.

Am I correct in using IN; would it be better to use ON?

In is better, I think.

The programme was on that day. It was on Channel 4. In the programme, an interesting conversation developed.

Thank you~