You're certainly not the first to ask that question, and it'll probably come back to haunt us if Forbes crops up again as a leadership contender.
THe TL;DR answer is that Blackford wears his religious opinions lightly, and has indicated in the past that he's not particularly conservative on social issues, supporting same-sex marriage and rights to abortion, for instance.
The slightly longer answer is all that - this (surprisingly) non-satirical blog post from 2019 challenges Blackford's views, how they chime or not with the run-of-the-mill Wee Freers and where his own personal limits might lie, these readers' letters to The National ask much the same question as you - plus a couple of other facts: (1) Blackford, when standing for high office, didn't feel the need to declare at the beginning of his campaign that his religion was at odds with relatively liberal modern values that are important to the current-day SNP, but he'd be able to ignore all that if elected; (2) he's an ex-investment banker, rather rotund and quite wordy on his feet, so there was plenty of other ammunition for his gainsayers.
For balance, since Yousaf's election, when he pointed to a voting record that was on the liberal side on such issues and has since shown himself to be a relatively secular though observant Muslim, racists the length and breadth of Twitter have branded him as some sort of jihadist maniac, perhaps not coincidentally because he's been very outspoken on the need for a ceasefire in the Israel-Palestine conflict, which wasn't surprising as for one thing he had in-laws trapped in it.
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