All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
The condition for placing ads and profiting of them in general is that they are ads for lawful things and do not, for instance, promote alcoholic beverages or immoral activities or what resembles such things (due to their evil). If the ads are for prohibited things, then it is impermissible to place them on the blog or profit from them.
As for earning money by placing ads on a website that specializes in religious content, there is no religious impediment to this, Allaah willing. It is permissible to earn money from making such a website even according to the scholars who held that it is forbidden to receive a fee for doing deeds intended to draw a person closer to Allaah (such as reciting or teaching Quran). They restricted this forbiddance to certain people, such as scholars and Quran tutors. As for other than them, it is not forbidden to earn money from that. Ibn Qudaamah said in al-Mughni, "As for building mosques, it is not necessarily done by a person who intends to draw closer to Allaah thereby. It may be done by someone who does not hold that intention (such as a non-Muslim hired to build the mosque). If the deed is done in return for a fee, then it is not considered done with the intention of drawing closer to Allaah or as an act of worship." [Al-Mughni]
So, if it is permissible to receive a fee for making a website or a blog that calls to Islam, then it is permissible to make profit by placing lawful ads on it with greater reason.
Allaah knows best.